You won’t be able to log in, post, comment, etc., with this account. Basically, the website lemm.ee is going to be gone forever. Lemm.ee is how you do stuff in the Fediverse. The rest of the Fediverse will still be up, and the instances you interacted with will have copies of your posts and comments, but your account will otherwise be dead.

So you probably do want to change instances.