You don't need to download the pro version, the home and pro setup packages contain the same executable, the difference between the two is that the pro parcel contains the files for the multilingual interface and additional languages for the spell checker, whereas the home package needs the international pack (can be downloaded for free) for this.

Using an encrypted message base in stead of a plain message base needs more.

Create a backup from within TB (Tools -> Backup)
Uninstall TB
When you're running TB from a non admin windows account, you need to delete the full HKEY_CURRENT_USER\Software\RIT\The Bat! tree from the registry (with regedit or something like that)
Delete the existing TB message base
Install TB again
On running TB it offers the option to use an encrypted message hase, check that option.
Next it asks for the mail directory, you can pick whatever you like.
Finally it asks whether tyo restore from backup or to create new accounts, pick restore from backup.

That's about it.
Oh and when you enable encryption, you've got to enter a password. That password needs to be entered everytime TB gets started.