It'd be extremely unusual for such changes to be retrospective. For example, if you are currently required to keep certain kinds of financial records for 3 year

It'd be extremely unusual for such changes to be retrospective. For example, if you are currently required to keep certain kinds of financial records for 3 years, and you delete records older than that, you won't be held at fault when that law gets updated to say 5 years, and you then fail to produce a 4-year-old document; it has not yet been a year since the new law took effect. This extends to criminal law too, for example with statutes of limitation. If, 20 years and 1 day ago, there was a 20-year statute of limitations for murder charges, you cannot be charged for committing a murder on that day (barring extraneous circumstances, such as tolling); even if a law was passed the day after said murder removing this limitation.