To summarize, On my system, I was having
1st user with uid=1000, gid=1002,
2nd user with uid=1001, gid=1003,
Groups network=1000 and storage=1001 were visible in /etc/group file.
I ran following commends from 1st user's account-
To change gid of network from 1000 to 200
sudo groupmod -g 200 network
Then
sudo groupmod -g 1000 <1st username>
sudo groupmod -g 201 storage
sudo groupmod -g 1001 <2nd username>
Thereafter, to change group of files from 1002 to 1000
find /home/<1st username> -group 1002 -exec chgrp -h <1st username> {} \;
find /media/data -group 1002 -exec chgrp -h <1st username> {} \;
(I think, instead of running above 2 separate commends, one can run a single command on /.)
From second user's account
find /home/<2nd username> -group 1003 -exec chgrp -h <2nd username> {} \;
I haven't changed gids of files from 1000 to 200 and from 1001 to 201.
Thanks for guidance and hope this will help whoever wants to attempt for this.