I know for a fact that the 2000 and 2001's are swappable.
When I got my car chipped a couple years ago, I was the last in line of about a dozen cars getting chipped. The way it worked was that the Upsolute guy would go down the line of TDI's, pulling the ECU's, then redo the chips, and then go down the line again putting the ECU's back in, one car after the other. Like an assembly line.
There was a mix up, and when they got to my car (the last in line), the only ECU left was for a 2000. My car is a 2001...
To make a long story short, I didn't put up a fuss, because my car had the highest miles of the bunch, and I figured I got a newer ECU than the one I had originally, even if it is from the previous year.
I've put on about 100k miles since then, and haven't had any trouble, other than my wife yelling at me to slow down... /images/graemlins/grin.gif