It is unlikely for larger instance to go down, but if it did go down, theoretically you should be able to migrate before the instance shutdown. Unfortunately, AFAIK Lemmy haven't implemented account migration yet: https://github.com/LemmyNet/lemmy/issues/1985

However, other fedi service like mastodon do have that feature, so it should work similarly. I imagine once it is implemented, you will be able to keep all your subscriptions (basically like exporting your communities, and automatically clicking "subscribe" on each one) and likely your account settings. But I don't imagine your post/comment can migrate to your new account.