The way it works on Mastodon is you essentially set up your new account on the new instance first, then you fill in the migration settings on both accounts. One to say where you’ve moved to, one to say where you’ve moved from. This adds a migration notice to your original account telling people who stumble across it where they can find you.
You can export and reimport all the important things like your bookmarks etc, but your posts stay behind on the old profile. Your followers / following is moved across, which in this case would be analogous to your subscriptions which are of course the most important thing.
I think a system like that would be perfectly fine here too. You don’t really need to bring all your posts over to a new profile, they’ll still be there and accessible on your old one if you need them.