The documentation page about transformations says:
It’s agile . When you modify your views, the changes take place immediately, for all future and past data, with no need to re-run the process over and over again. You can iterate quickly and get the results you need in seconds.
If you run a report this year that shows data from each of the past 5 years, will those past 4 years still match the report you ran last year? Or is there a chance that changes in transformations will lead to slightly different counts or summaries?