How to change the subject line of an existing message in the In box so that .eml file will have a new name
I'm trying to migrate emails from one hosting company to another. I'm using Thunderbird to download the messages from the former company using POP3, dragging and dropping each message to a folder on my computer (on Windows 10, I can't select them all and drag and drop). Each message becomes a separate .eml file. I then will use an IMAP connection to the new hosting company to upload the files from the folder on my computer.
When I have two messages with subject lines of the form Blah Blah Re: Blah Blah
in the drag and drop, Windows is treating the .eml file names as the same and presenting me only with "copy and replace" or "don't copy" alternatives. In Thunderbird, I've tried Message>Edit as New Message and changing the subject line, but that doesn't end up changing the subject line.
ప్రత్యుత్తరాలన్నీ (1)
I don't understand why you're copying them out to separate files. The POP3-connected account contents will persist even after the account has been terminated. All you'd need to do is set up the new account in Thunderbird then drag-and-drop (or use right-click|move to) to move the messages to the new account, if that's what you need or want to do. Some users seem happy to retain their "dormant" accounts in Thunderbird.
It's all email so is best kept within the email domain. Exporting means transforming the messages to text files and then back again from text to email afterwards, with all the attendant file-naming issues.