I'm in the same boat of early carelessness with my keys and these are my key rotation musings so far.

I wonder if any clients manage follows by nip05 instead of npub. That would allow relatively easy key rotation that is seamless to the followers.

There seems like an opportunity here for a rotation nip too. Publish and sign your new npub from your current nsec as a specific rotation event. Then clients can spot the nip314 event (no idea if this is taken I just used pi because rotation like a circle) and offer to refollow. This should not be automatic due to the abuse potential in the case of a beach.

The one thing I know to exist now that could help with planned key rotation is shipyard for scheduling posts. You should be able to dual post your normal content with automatic regularly scheduled "follow my new npub" posts on the old keys.

Follower counts are a shitcoin anyway right. This makes your followers proof of work for your high quality content.