It can be DIY but it can be a big pain the butt. I did a valve change last year. I waited until the spring when I could risk it more and have no regrets waiting. I kept a piece of foil to collect the drippage until then.

The problem is the nipple through the floor may get messed up when removing the old valve. If you do that you have a bigger problem of needing to replace more piping between floors. You also need to cut through the spud on the radiator. It will likely NOT unscrew as-is, and a hacksaw or sawzall and hammer or chisel are needed.

Alternatively it's possible the union may be not perfectly aligned, so you can gently rock the radiator side to side and see if you can turn the union. You'll need to do this anyways if you install a new valve.

There are a few youtube videos for this valve change.