How best to remove ventilation brick in bedroom wall?

[twoleftfeet]

How can I best remove the ventilation brick shown in my photo? It's in a bedroom and I want to replace it with a sliding vent. There are no fireplaces or gas appliances.

I'm worried I might cause too much damage to the wall if I try to chisel it off!

 

[noseall]

It looks like a louvered plaster vent screen. It should chisel off easily. Try hacking behind it with a brickies trowel, keeping the trowel flat against the wall. Once you get some purchase lever with the trowel. This should minimise any denting risk. Score around it with a knife first.