There's no way to change email service providers in Mojave without data loss

So, thought I'd post all this out for anyone who's trying to change email providers in Mail for Mojave. Hopefully you'll avoid the ten to twelve hours of work it's taken me so far to fix Mail's default behaviors.

Yesterday the MX records updated from my new email service provider, which was all well and good - I thought as I merrily changed my POP and SMTP settings in my accounts in Mail. Imagine my horror, tho as Mail decided to merrily begin deleting all my old emails - Sent, Inbox, Draft, you name it. Luckily I was able to hit Take Offline before all my work for the last four years was deleted but it's hard to understand why this is the default Mail behavior without so much as a hint that changing providers will nuke your entire mail database.

Luckily between my backups and taking the accounts offline I was able to mostly put things back - after hours of work. I decided my problem must have been that I didn't create new Accounts instead of just changing the SMTP and POP. Well, now Mail decides I can't create new accounts in order to avoid mass email deletion because "account already exists." Even though the new accounts have different SMTP and POP and different passwords.

So now my only option is to move all my old email out of the mailboxes so Mail won't delete it when I put in the new POP and SMTP settings, right? Well, of course I can't move email because my old accounts are offline since the MX has been changed.

My savior turns out to be a third party program called MailSteward which will grab the emails out of my active inboxes - but I cannot for the life of me figure out why the defaults for Mail are so unnecessarily ridiculously difficult for end users. I can't think of any way you'd be able to migrate mail providers without data loss or use of a third party program.

I'd have had to move all my email out of the boxes while they were still with my old provider, but how would I do this with no way of knowing for sure when the MX records would update?

Seriously, I can't create new accounts to prevent the MASS WHOLESALE DELETION OF MY ENTIRE EMAIL DATABASE because there's already an account with the same name even though it has different POP and SMTP settings and a different password?!

This is insane. I've been a Mac user for two decades but I'm sure sick of the Mail program. It can't move more than a few emails without getting stuck; I have to keep an elaborate system of mailboxes because it can't manage more than a couple thousand emails without crashing.... and now there's no practical way to change email providers without data loss. It's time for a rewrite of this program from the ground up.