This change goes hand in hand with monster leveling, which is also coming in the next patch.
In the next patch you will unlock monster forms by hatching eggs in the new Incubator Room. So don't panic when you see that you can't switch to another monster because it says that you only have 1 monster unlocked. To unlock the rest, just head to the Incubator room and hatch them all. Once unlocked you can freely switch between monster forms using the MChanger(blue computer) as normal.
Read more about how Egg Hatching will work here:
https://mdotexe.com/2016/08/02/time-to-make-some-eggs/