upgrading Thunderbird 32-bit with T.B. 64-bit

What is your operating system? You should not install a 64-bit version directly over a 32-bit version. Most operating systems will make it difficult for you to do this, for your own safety, and you shouldn't try to circumvent its efforts. It's best to uninstall the 32-bit version and then install the 64. The uninstall will leave your Profile, which is the folder that contains all your email accounts, messages, filters, preferences, etc. The new install should find and use your current profile with no problem. If it DOES have trouble finding it, that can be fixed. The 64-bit version will have no trouble using a profile created by a 32-bit. There's no disadvantage to "upgrading" to 64-bit (unless, of course, you're using a 32-bit operating system.) It would be a good idea to make a backup of your Profile before making the change, just in the unlikely case you encounter a problem. Actually, you should consider doing regular backups if your email collection is precious to you. Here's an article about how to do a backup. I recommend that you use the manual "Creating a backup of large profiles" method described midway through the article: https://support.mozilla.org/en-US/kb/thunderbird-export