Clients can simply accept both old and new addresses.
That's granted, but what should clients display? If they display old, it will be confusing, plus no benefit from checksum protection when users work with old addresses.
If they suddenly display new, it will also be confusing, because now all user's old addresses have changed.
And it seems nobody will bother to update NRS, because it's being phased out
EDIT: How will this affect API calls?
Since both old and new addresses are accepted and converted into the same underlying account ID, there should be no problems, API's should work with both transparently.
That's for input. For output, it's a different story. But for now, I guess, the API can return old addresses and hope they won't get corrupted on its way to the user...
Or the API can be duplicated, similar to DirectX, to support both old and new versions. Something like:
localhost:7874/nxt?requestType=getAccountId&secretPhrase=PASSPHRASE
vs
localhost:7874/nxt2?requestType=getAccountId&secretPhrase=PASSPHRASE