Merge old email account folders into new email account folders?

My profile contains 30 years of work emails which I sort as received into organized folders, stored on my computer. This archive is used frequently to search re projects, correspondence. My email provider is shutting down. I have already set up my own POP3 server, with new email address. I'm using webmail until I'm confident I can make the transition w/o screwing up! [Had one storage melt-down which required weeks of work to recover folders from a backup. Once burned, twice shy.] >> How do I set up/alter Thunderbird profile and/or folders so that emails coming in to NEW address go into the existing inbox/profile folder(s)? I want to be able to search both NEW and archival emails w/o having to switch between folders/accounts/profiles. Bonus Q: The old provider is shutting down 12/15/26. While my correspondents adjust to my new email address, being able to see/process emails sent to the old address w/o changing profiles, etc., would be great. Thanks!!