IOS does not allow selection of downgrade to 5GB

Trying to downgrade from 40GB to the free 5GB is apparently not allowed for IOS. I check that option, the checkmark appears to the right, but when I tap "Done" nothing happens. I can apparently go to 20 GB for $20. 5GB is free for the Mac, why not for IOS. It is the same storage. Do I have to pay different for the two systems or pay double or whatever? I wish they never threw out the old iCloud system where I could put stuff there in files organized like the Mac and use them. The way now is really clunky and there is no ease of use.