I tried it the way you described and it worked perfectly !
Thank you so much Kasia, this will save me an enormous amount of time.
Here is my suggestion how to invert target and source languages in TM:
1. Open your TM
2. Export it to a text file (in the FILE menu)
3. create a new TM and set the language direction to the opposite of the exported file
4. Import your old memory
5. Reorganize (in the File menu).
I hope it works. Success!