Just got off the phone with Apple support:
You CANNOT ENABLE IT BACK BUT RE-ADD IT BACK!
So do not disable devices if you think you would want to enable them back, cause DISABLE DELETES your device from provisioning profile! ENABLE - ADDS it back AND USES FREE SLOT, if you have any.
They promised to clarify the language used so it wont be so misleading.
!!!EDIT!!! That is really something !!!
Actually the apple support was WRONG! Since I had all 100 slots used up they gave me, as a curtesy 10 more, so I could enable some devices back and as they said they would occupy those empty slots.
I did enable 2 disabled devices, but they DID NOT USE Empty slots!!! They just appeared enabled as supposed to. So it means their database is doing something wrong when enabling and requires empty slot to proceed! probably it creates a copy and delete disabled one or something like that.
So keep at least 1 empty slot for enabling disabled devices.
That really makes you wonder wth Apple, really? Such wack bug?