Thanks for all the comments.
Seems I did everything right so far, stored in a dry clean cellar, always around 15 centigrade, dry, upright in a disk box. I'll just put them back there then.
Just for clarification:
Main problems are humidity and adhesives, not demagnetization?
But still: will it do any harm to a currently good disk to rewrite it?
If you try to re-write data to "refresh" an old disk which is shedding oxide, you'll probably end up with a worse problem (as every time the drive head touches the disc surface it will do more damage).
I already read all the disks, so the drive head was already there and the disk seems to be good.
So in this particular case, rewrite or not? Question is simply when will natural demagnetization hit? If it's in 100 years on average, I obviously don't want to rewrite them, if it's 30 years on average, I probably want to rewrite them.
Besides, I just made 20 copies of the original Workbench disk, hopefully one survives.