This is sort of how it works.
24 Word Phrase -> Address 1A
-> Address 2A
-> .... (infinite A addresses)
New 24 Word Phrase -> Address 1B
-> Address 2B
-> .... (infinite B addresses)
So in order to access a specific address you need to set your Ledger to use the 24 word phrase for that address.
If you want to restore your previous addresses / word phrase then send all funds in your current set of addresses to one of your previous addresses. Then restore your Ledger from backup and use your previous 24 word phrase.
Whichever word phrase / addresses you aren't going to be using in the future, you can manually type it in on MEW as well without it actually being on your Ledger. However, and I cannot emphasize this enough, **DO NOT UNDER ANY CIRCUMSTANCES CONSIDER THE ADDRESSES UNDER A 24 WORD PHRASE "SECURE" "OFFLINE" ONCE YOU HAVE TYPED IT IN MEW. Only do this if you need to migrate funds OUT of a word phrase and then do not use that word phrase again (or understand fully that the core features that make the Ledger Nano S secure have been bypassed for this key)