As mentioned, you have to push the black-circle-buttom and lift/pull the plate with some force.
I was able to remove all the plates on my ProWins.
If you want to install Magpul USGI Ranger Plates, you have more mods to do
I sanded the sides and wings off of the plastic plate lock and round the front corners to make it fit. Then I also sanded a bit of rubber off of the bottom of the rubber ranger-pull. Its a tough fit, but it works.
If anyone needs any help/tips on the Magpul USGI Ranger Plates , feel free to PM me or post here.