DNA change in the same persons?

The earliest finds of human remains  in Western Europe involved the Western Hunter Gatherers (WHG).
Samples of Y-DNA extracted from these individuals belonged exclusively to R haplotypes (particularly subclades of R1b1) and I haplotypes (particularly subclades of I2). mtDNA belonged almost exclusively to U (particularly subclades of U5 and U4).
Other studies gave different results.

After that came the Early Europea farmers (EEF). The Anatolian Neolithic Farmers were an ancestral component, first identified in farmers from Anatolia (also known as Asia Minor, Turkey) in the Neolithic, and outside of Europe and Northwest Africa.
The highest EEF ancestry found in modern Europeans ranges from 67% to over 80% in modern Sardinians, Italians, and Iberians, with the lowest EEF ancestry found in modern Europeans ranging around 35-40% in modern Finns, Lithuanians and Latvians.
The  genetic makeup of the EEF included paternal haplogroup G2a and common maternal haplogroups like H2, I, and T2, but also some East Asian lineages in certain regions. Over time, these farmers mixed with local hunter-gatherers, contributing significantly to the gene pool of modern Europeans. 
The EEF were quite short (ca. 5 feet) at least at first.

Then came the Bellbeakers.

Then came the Bellbeakers and the Bronze Age. In France, Britain, and Ireland Bellbeakers with R1b Ychromosom replaced 90% of the population within 100 to 200 years. Ireland was influenced after Britain. Bellbeakers at first replaced 90% of the males. They were nearly all R1b. Before then R1b had been present among the Hunter-Gatherers but in small quantities. The early Bellbeakers were roundheaded and robust.

Terminology Confusion: Bellbeakers, Steppe Ancestry, R1b, are all overlapping terms but not completely so.

Etruscan, Iberian, German, British, etc, groups all show intrusion of Bellbeaker types. "This Bell Beaker-related steppe ancestry was in turn derived from the earlier Corded Ware culture, rather than from the Yamnaya culture. " This however may be mistaken.  Greece, Sardinia, Sicily, also received minority inlfuences from the Bellbeakers and so did Ancient Egypt.

Corded Ware Culture was mixed R1b and R1a whereas for Bellbeakers it is all R1b. Therefore it is unlikely that they originated from a Corded Ware civilization.

East of the Bellbeakers in Poland and its neighborhood a similar event was taking place. Other there the dominant party was not R1b but rather R1a. Just as R1b took over on the male side in the west so did R1a in the east. Originally in Poland (Corded War territory)  the male haplogroup I1 had sominated, then it was repalced by R1a, but later I1 came back and dominated again (in the Late Iron Age) only to once again  be replaced   by R1a "returning".

Bellbeakers in Britain took over 90% of the genome within 100 to 300 years. Bellbeakers in Britain and Ireland had  10 males for every female or almost no females at all!

"approximately five to  14 migrating males for every migrating female.” 

What happened to the R1b females in Britain and Ireland? What happeneded to the Neolithic males whom the Bellbeakers replaced? The Bellbeakers were very numerous and widely spread. Where did they all come from? Where did all their women go to? There is Little evidence of warfare.  Where did the neoliythic malers all go to? We Suggest they all stayed where they were, or they mostly did. There was a change of DNA from Neolithic Types to R1b. Cultural Continuation from the Neoilthic, continued use of family graves. Alongside many changes.

And what about Poland? Why and how did the same change take place but only with R1a?

When did this change take place? Our estimation is ca. 300 BCE. Conventional dates however place it at:  ca. 3000 BCE!

Sources:

^^^^^^^^^^^^^^^^^^^^^^^

The Origins of Haplogroup R1b: How Steppe Ancestry Changed Europe Forever…

https://www.youtube.com/watch?v=58gT4lYREBo&t=55s

The Beaker Phenomenon and the Genomic Transformation of Northwest Europe
https://pmc.ncbi.nlm.nih.gov/articles/PMC5973796/

However, migration played a key role in the further dissemination of the Beaker Complex, a phenomenon we document most clearly in Britain, where the spread of the Beaker Complex introduced high levels of Steppe-related ancestry and was associated with a replacement of ~90% of Britain's gene pool within a few hundred years, continuing the east-to-west expansion that had brought Steppe-related ancestry into central and northern Europe 400 years earlier.

Regardless of the geographic origin, by 2500 BCE the Beaker Complex had spread throughout western Europe (and northwest Africa), and reached southern and Atlantic France, Italy and central Europe5, where it overlapped geographically with the Corded Ware Complex. Within another hundred years, it had expanded to Britain and Ireland8. A major debate in archaeology has revolved around the question of whether the spread of the Beaker Complex was mediated by the movement of people, culture, or a combination of both9. Genome-wide data have revealed high proportions of Steppe-related ancestry in Beaker Complex-associated individuals from Germany and the Czech Republic2-4, showing that they derived from mixtures of populations from the Steppe and the preceding Neolithic farmers of Europe.

The Y-chromosome composition of Beaker-associated males was dominated by R1b-M269 (Supplementary Table 4), a lineage associated with the arrival of Steppe migrants in central Europe after 3000 BCE2,3. 
Outside Iberia, this lineage was present in 84 out of 90 analysed males. For individuals in whom we could determine the R1b-M269 subtype (n=60), we found that all but two had the derived allele for the R1b-S116/P312 polymorphism, which defines the dominant subtype in western Europe today14. In contrast, Beaker-associated individuals from the Iberian Peninsula carried a higher proportion of Y haplogroups known to be common across Europe during the earlier Neolithic period2,4,15,16, such as I (n=5) and G2 (n=1), while R1b-M269 was found in four individuals with a genome-wide signal of Steppe-related ancestry (the two with higher coverage could be further classified as R1b-S116/P312). Finding this widespread presence of the R1b-S116/P312 polymorphism in ancient individuals from central and western Europe suggests that people associated with the Beaker Complex may have had an important role in the dissemination of this lineage throughout most of its present-day distribution.

While Y-chromosome haplogroup R1b was completely absent in Neolithic individuals (n=33), it represents more than 90% of the Y-chromosomes during Copper and Bronze Age Britain (n=52) (Fig. 3). The introduction of new mtDNA haplogroups such as I, R1a and U4, which were present in Beaker-associated populations from continental Europe but not in Neolithic Britain (Supplementary Table 3), suggests that both men and women were involved. 

 ^^^^^^^^^^^^^^^^^^^^^^^

Scientists Reveal Surprising Genetic Origins of The Bell Beaker People

https://www.youtube.com/watch?v=5CFWcJR6ysg