Maybe someone was in the same Situation as i was:
I use Thunderbird for a verry long time and til this Morning, i had the Windows 32-Bit Version on my Win10 x64 System installed.
However, the new Default Installer of Thunderbird for Windows is targetting Windows x64-Systems (but the Installer remains 32-Bit).
Long story short:
To Migrate from Thunderbird 32-Bit to 64-Bit for Windows, you just need to download the default installer from thunderbird.net
and click & run it and it will automatically migrate your Data and profiles without further interactions.
So, it was a surprisingly smooth process without any complications.