Thank you ShadySue.
Well, my concern was about possible issues with reviewers, but it seems from your words that there are no such problems.
The matter is:
I have a little portfolio in IS, less than 50% of which is in PP.
One day I added a relevant keyword to an accepted image: the day after that image popped out in TS. Yesterday I tried another images: approved, in portfolio at IS but not in PP. Today that image is in PP. It seems that the trick works, and maybe other contributors can force their images in PP in this way.
Perhaps modifying title or description works too, or just opening the image for editing
and saving without modification.