The "old" rules settings are still there and your sieve code is unaffected if your additions are at the very beginning or end.
If you are running with the "new" rules UI then you can create rules like you did with the "old" UI by clicking "Switch to no preview rules", creating a condition (or conditions) in the "old" way and then clicking continue. All the "old" options are still there.
"Old" sieve code you may have added to the beginning or end edit blocks is preserved and should still work as before. Stuff in the middle two edit blocks of the "old" sieve code isn't preserved (not sure why the one before section 4 isn't since there's also an edit block before the "new" section 4, different context I guess). I suspect if all you are doing is adding additional filters you probably only added them to the end block in which case all those should still work there unchanged.
FWIW I don't think I will ever be using the "new" default way of creating rules through the UI (always using the "no preview" mode). The default rule creation is too restrictive, takes too many clicks to navigate it, I believe uses "
stemming" (useful possibly for searches) to match so the matches are not necessarily exact, and once you create a rule using it you can
never switch that rule to using "no preview" mode. Indeed, since my account is old enough that I have the choice of "new" UI and "old" UI (I've read here new accounts don't have that choice) I haven't even switched to the new UI except for the times I wanted to see what changed in the sieve code and if those changes would affect my sieve code additions. I'll stick with the "old" rules UI until the choice is taken out of my hands.