Here's a canned response...

You can switch to pro without doing anything, you don't even have to
install again, because the executable in both versions is the same,
the pro features are being unleashed by the pro-key, not by the
install file you've used. The only difference between the two
installation files is that the pro file contains the international
pack, while the home version needs that to be downloaded separately.

However, when you want to switch to OTFE (On The Fly Encryption), it's
not that easy. TB cannot encrypt an existing unencrypted message base,
It can only start with OTFE while you're still without message base.
What this means is that in order to switch to OTFE, you need to make a
full TB-style back-up. Afterwards you need to uninstall TB. And
delete/move your message base. (Last time I uninstalled TB left the
message base in it's place, just as I think it should)
Next you install TB. At the end of the install or the start of TB's
first run, it'll ask you about OTFE and you enable that, the password
you're giving then is important, because without that password your
data are lost forever. Amd on the question whether you'd like to
create a new account or restore from backup, you restore from backup.
After restoring from backup you quit TB and restart it (otherwise your
restored preferences won't be loaded).
That's about it.

Oh yes. When you're using TB in multiple user accounts in Windows, you
need to realize that to change another account than you used for the
uninstalling from plain to OTFE you'll need to remove the full
HKEY_CURRENT_USER\Software\RIT\The Bat!
tree from your registry in that account. Because the uninstall routine
only deletes that for the active account during install.
(Yes, actually that means that you don't have to uninstall. Just a
backup and removing both the message base and the proper registry
entries will prompt TB to ask about OTFE too.)