Updating baseboard radiator
Can't tell from looking at the picture you posted but it looks like a finned tube hot water baseboard unit. Do you heat your house with a hot water boiler and if not can you elaborate. If I am right and that is a hot water unit, the installer recessed the enclosure into the wall and mounted it directly onto the wall studs. I never liked that type of installation and would prefer the unit to be mounted on top of the finished wall.
Finned tube baseboard has aluminum fins, usually 4-5 per inch slid on a copper tube. Looking at the picture it looks like the fins could use a good cleaning with a shop vac and a very soft brush. The fins are very delicate and sharp so be careful. As far as the outer cover you could refinish as you wish. As far as replacing them, that would be a major project probably left to an HVAC company.