From my understanding S/MIME is pretty much enterprise PGP, so if you don't have your old certificates you can't access old emails that were encrypted using them
Encryption is performed with private key, not with certificate. If you can issue new certificate for the same private key, it should be able to decrypt old stuff.
Rotating keys might be a good security practice, though. But not necessary.
i've always reencrypted when getting a new cert. not sure if thats an antipattern though. i just didn't want to be bothered by which cert encrypted what data
