What do you mean by corn plus variable driveIJ I had both a Corn Plus and a Corn Soybean Special and as far as I can remember, the drives in both machines were the same.
I may be mistaken Tom, but I think both of your combines have the variable sheave on the left side of the machine that you plug the hydraulic hose into to vary the speed of a row or corn head because they are both corn models. But a lot of machines are like this one of mine that is not a corn model and just have a fixed sheave, whereas yours can move in and out. They were meant for cutting wheat and the like. As far as differences between corn plus and corn and soybean special - I don't suppose there is any, at least on the sheave part.
the vari speed can be switched, all sheaves and idlers get changed, as well as the front counter shaft, and belt.Then you need to address the hyds. Valve, hoses,wiring, and re-wiring the switch. As for other differences,there are plates on the back of each walker, plus a corn kit, that installs into the back of the machine to protect it from cobs from the chopper.
It should not be that big of a job,you will have to change lower pulley and shaft and upper pulley with stub shaft along with idler.The upper stub shaft is what the feeder housing hangs from,so you will have to support it while changing things.The hyd. valve is the same for the variable speed reel and the variable speed header drive,I don't remember for sure but I think the lines are just hooked together,to use it for reel you just uncouple at header drive pulley.