If you own your own domain name for your email, then it's pretty simple and will take a few hours of effort to get fully moved over.

Part of why I pushed myself to move away from Gmail was I was still using the @gmail address occasionally.

Luckily, I'd been using my own domains for a long time for most emails, so it's been a pretty easy transition - I still have Fastmail poll Gmail for emails to make sure I see the ones that still go to @gmail.

It is not a lot of effort. You can keep receiving and sending to/from your old e-mail within the new provider for as long as you please.

I am in the process of doing that, and my answer has been to move all the things that I can to the new account, and then keep the old one around until nothing more I care about comes there.

And you can be I own the domain for the new email. Not going to make that mistake again.