Come on guys – we've been doing perfectly acceptable green screen for years with composite video, and then HD. Some people may well be leaving HD for 4K, but they are NOT representative of the entire industry. It's true that the DLSR people have moved fast, and now the large sensor enthusiasts are going up again, but they are still pioneers. So much still ends up on DVD in SD, and we're still watching TV programmes recorded before HD even existed. What matters is sharp edges, and decent lighting. The 4K people often have real issues because of the soft edges that accidentally happen when DoF is very shallow.
First thing is to try your old camera with something like Premiere or Vegas or any of the proper editors and see what the results are? The keyer in Premiere is the one developed by serious magic, and was an incredible SD keyer – Adobe took the best bits.What you need to do is get space between you (the subject) and the green background. Proper chromakey green or blue paint is a very 'pure' colour, and it looks vivid, but ordinary green emulsion is fine if it is even, and the same colour. You need to light it so as the brightness is consistent – no hot or dark spots, and then make sure the subject is far enough away the green doesn't reflect onto the edges of the face and body. Your subject and screen lighting is separate – then, when the camera is adjusted to make the focus as sharp as you can get, even an old camera will key pretty well. Mess up the lighting or focus and it will be awful. I've been keying in 4:3 and 16:9 for a long time in SD on DV tape, and very rarely do I get problems that are down to the format – in the same way the BBC and ITV did before HD. Bad keying is usually bad lighting and setup – artiste too close to the screen, soft focus etc. The often commented on things like 4:2:2 or 4:2:0 etc are differences AFTER you got the lighting right. I don't shoot SD any more, even though most of my work ends up on DVD, but when I key greenscreen, on HD, it's not really much different to SD. The reasons given above are not wrong – just over emphasised, and what you are propsoing is what thousands of students on media course do – and it's perfectly possible with a proper editor. Go for it, and just be prepared to spend time on the setup and lighting. You really need about a couple of metres between the green surface and your subject – have you got this?