I've had this same experience some time ago, but now I know a lot more. Thunderbird creates a Profile folder and in it is a mail folder. What I'm not too certain about at this moment is if by creating your new account that you also created a new profile folder. That has likely happened. Whether or no, I'd suggest opening your current Thunderbird account and under both local folders and the main account area create one folder each with an odd name. Here's an image of this accomplished in Local Folders:
This is strictly for identification purposes so you know you've got the right folders. I'd choose something near the top of the alphabet.
Aardvark and AARP come to mind.
Once you have done that, close Thunderbird.
Now, here's where you get to navigate.
My computer ==> Local Disk ==> Documents and Settings ==> Your user name on the computer (woodman?) ==> Application Data ==> Thunderbird ==> Profiles ==> Mail
If you see more than one profile, open that profile and keep that window open, then do the navigation over again and open the OTHER profile. Resize your windows so that they are either side-by-side or one above and one below.
Now, in each window, open the Mail folder. Inside that folder in each window, you will see a folder labeled Local Folders and a folder with your account name. In One window, open the Local folder. See if your odd named folder is in it. If it is, then you have your new account. If it isn't go back to the previous folder for now.
Once you have the New account local folder open, then open the account folder for the old profile.
To avoid possible mail loss, RENAME the Sent folder (it won't look like a folder but it is) and the Inbox folder in the OLD account folder to Sent-Previous and Inbox-Previous. If you have any other folders in the OLD profile that match any folder names in the NEW account, rename them in a similar fashion. Please note you do not need to do this with the folders with this on them .msf Once you have finished renaming the folders, then move the folders that DO NOT have the extension .msf To the NEW local folder.
Once you have finished that, in the OLD profile go back to the previous window and open LOCAL folders. Again, rename anything that matches a name in the NEW profile, and move the folders that don't have .msf to the new LOCAL folder which should still be open.
In either step, if you see the extension .sbd, transfer that over as well from the old to the new, again renaming anything that would match a name in the New folder. Those are the folders containing folders in Thunderbird.
Here's picture of an open Local folder with the various kinds of files and folders you will see in it.
I hope this is clear to you, if not, please ask and I'll clarify.