Can you use a usb key that works with ETS5 on ETS3?

Hi,

I recently started using ETS again, and purchased version 5 professional. (if I'm not mistaken)

I remember going throug some procedure to move or copy the license key to the stick.

Today, I'd like to step through my old ETS 3 course (2007 - 2009).

For this it would maybe helpfull to install ETS 3 (I still have an elderly laptop with Windows XP that I also use for Simatic PLC software). So I searched and found Vassilios Lourdas post with the links to ETS 3 and ETS 4. Perfect! Thanks for that!

Now my question:

How easily can I use my Key to back and forth between ETS 3 and ETS 5?
Or would it be easier to purchase a suplementary key?

Thanks for helping out!!

Erwin