If Damus works like all other clients then the followers list is published to the relays and will be found no matter what client you use.

I have personally had some ux bumps switching around clients in the earlier days, but recently I've switched around without problems.

All nostr clients share the same list of followers. If they are missing that is a technical issue, e.g. if you are not connected to the same relays.

You need to use the same private key that you have created with the first app you were using.