There is generally only one 'account' named Local Folders, and it should not be renamed. When it's selected in Tools/Account Settings, the right pane appears as in the attached picture, which is different from regular mail accounts. Maybe things got mixed up when you downgraded from the beta to the release 68. At this point in the development, the beta and release should use separate profiles.

Here are some screenshots I took to illustrate the issue I'm experiencing. The first image displays some emails from a while ago with "Local Folders" shown in place of "Gmail." This is to illustrate that the names of my other email accounts changed successfully for all email received from those accounts, even in the past. The second image shows my different email servers with the names given to each account. The third image shows that I am receiving email now with the correct account name, "Gmail," but older emails did not retroactively receive the account name.

After I uninstalled the most recent beta version once I experienced some nasty errors with it (email headers were not showing up properly and the address book was completely broken), I copied my email folders to another directory, deleted all local and roaming data in the "Users" folder and created a fresh profile. I did end up copying the email folders into the mail directory since I could not come up with a more elegant way of saving them, but I added new account and preference data from scratch.

Did you use a Global Inbox before? That would place mail in the Local Folders 'account'.