Found an approach, enter scontrol interpreter (in command line type scontrol) and then
scontrol: update NodeName=node10 State=DOWN Reason="undraining"
scontrol: update NodeName=node10 State=RESUME
Then
scontrol: show node node10
displays amongst other info
State=IDLE
Update: some of these nodes got DRAIN state back; noticed their root partition was full after e.g. show node a10 which showed Reason=SlurmdSpoolDir is full, thus in Ubuntu sudo apt-get clean to remove /var/cache/apt contents and also gzipped some /var/log files.