I now use Audacity 3.4.2. on Mac.
When it updated, I did see a popup I didn’t quite understand, saying my projects would get resampled and I may be losing stuff on tracks so I had to check (sorry, I don’t have the correct wording)
I think what happened is that all the tracks I had mixed down from stereo to mono became stereo again, except as 2 split tracks.
What I did then to “simplify” the project was to ‘mix and render into new track’ one of the tracks from the split stereo, and I did this on all tracks 
What I didn’t realise, is that it was mixing down as a hard pan (Left).
Now all my tracks are mono tracks hardpanned left, no sound on right, and it seems there is NO WAY to recover a centered panning!! "Make stereo track’ is greyed out, and if I copy any track into a new stereo track, it is only 2 tracks on the left.
It is a fairly large project, with over a hundred names clips for an audio doc and unless I am able to make all tracks centered (pan), I will have lost everything. 
Thanks for your help!