Well Infinite Volumes are probalby not the path you want to go down since they have been deprecated in favour of FlexGroups, and as of 9.6 there is no in-place conversion. Talk to your local SE about this if you have a use case, perhaps the feature will gain traction and priority.
I asked at last year's Insight conference when 9.5 was announced and was told that it's on the roadmap but it's not there yet.
You can certainly do it by taking the volume offline, but that's not what most of us want.
The one use case that's a problem for me today is SnapVault destinations - I had a 70TB volume that couldn't SV any longer because the destation - with its snapshots - hit 100TB. There's nothing you can do since you can't SV from a FlexVol to a FlexGroup.