If you buy your 2017 iMac with the built in VESA mount and decide later to remove it and use the stand, it will require a complete disassembly of the iMac. It's really a difficult job to undergo and not recommended unless you are adept at doing that sort of thing. As for the iMac Pro VESA mount, I have no idea but perhaps someone else would know.
The VESA mount iMac models are actually separate models, and don't ship with the normal stand.
I think that the VESA adapter could be swapped out, but is a major surgical procedure - the stand, or the VESA adapter, are mounted from inside, and require removing all inside components to get at the mount screws (but don't assume that it is possible, as I am not even sure about that)
The new iMac Pro will take the adapter similar to VESA adapter plates on other displays - from the outside.
NOT useful, or "adaptable" on standard iMacs.
Maybe next year, when other iMacs shift to the newer case (thus making even RAM upgrades on "normal" 27-inch iMacs very challenging, if it is still possible at all)