I'm assuming your's is a HST tractor. I don't have a 20, but my 25 (and I think all of the HSTs) is set up so the the hydraulic pump is allowed to have more stroke (which means more speed) in the forward direction. You can look at the decal on the tractor or the owners manual to confirm this. I haven't had my pump out so I don't know if it is possible to reverse the forward and reverse ports when installing it or to set the forward stroke to what is the reverse stroke setting, but it sure sounds like that is what happened when they reinstalled your pump.