It isn't so much Thunderbird, but IMAP, which wasn't designed for high-volume transfers. That is, such a move can be probematic, and some data loss can happen. I suggest a four-step process: first, create a filter to COPY messages by date range to local folders. Once content with that process, then redo filter to delete those messages from server. Third, create a filter to upload local folder to new account (the filter's purpose here is to slow down the copying process. Finally, after verifying that the copy was successful, delete the local folder. TIP: If either account has AllMail folder or Important folder or any others that contain redundant messages, be sure to UNSUBSCRIBE from both accounts.

Before proceeding, I suggest waiting a bit to see if other suggestions are posted. Others may have more preferred ideas.

David,

Thanks for giving me a beter background about IMAP's capabilities and your suggestion for a possible solution.

I'll await other replies and start by migrating my emails with large attachments, since it doesn't appear that Thunderbird can accomodate my request. I'm surprised about that since I thought for sure that this would be such a common issue. Dan

Migrating emails from one Gmail account to another Gmail account while specifying a date range can be achieved using Google Takeout and Gmail Import.