Someday I will learn how to replace these things and put in some better quality equipment...
Replace them with 1/4 valves when you do.
If they're threaded connections, most people that can "turn a wrench" could replace them. Obviously remember to turn off the water supply (you'll still get some "spillage" but nothing you can't clean up with a towel), use sealing tape (on metal pipe), and crank them down very firm.
Oh yah, definitely would put quality quarter turn valves in...I can't imagine it would be that difficult, but there are so many other things with our house that more urgently need attention. The wife has been complaining about her shower dripping and I was not able to get things apart like I was able to do pretty easily with my shower when it dripped when new, so I have to figure out what the next step on that might be (short of paying a plumber tons of money). Plus the sunroom was built by guys who didn't give a hoot because the contractor was in the hospital for the most critical part of the build cycle and we have all kinds of issues as result (and the contractor is now deceased, sad to say)...you get the picture!!