So far, here's my experience with the migration:
Relatively smooth. Here were the problems:
1. use_ssl was not enabled in DirectAdmin. Many of the SSL certs were failing and reverting to self-signed certs. The process to correct it was to enter each DA User, make sure the DNS settings were correct with A records for mail, pop and smtp, then change the TTL settings to 600. Wait a few minutes and then re-create the SSL certs using Let's Encrypt, checking off the domain, www, mail, pop and stmp.
2. Email issues - Apple and Outlook users are having a heck of a time connecting again. Working with them one by one to get it sorted.
3. Evidently, there is no way for DirectAdmin to change an email user's password from the control panel. While DA is pretty simple for what's there, I'm finding trying to reset the email account password definitely a problem. You can't even do it from the Users DA panel. The DA knowledge base has an article from way back with an old plugin to manage email passwords. Really? Can't believe that's the only way.
KH support is doing fine with all of these issues, of course, with their usual quick support. But I wanted to pass along the things to check (each account's DNS and SSL cert), and to be prepared for email issues.