Changing the tub is not that hard. The only problem i see you having is that you might not get the mounting gear with the tub. Rubber mounts and correct bolts.
Also if anything has been mounted under the tray might be in the way when the tub goes on.
If not grab a few mates and a carton and it will be all done before you know it. The only real tricky bit i think would be the fuel filler (may have been changed with the steel tray) and then getting the lights to work.
i recently changed my damaged tub for a brand new one, easy as to swap, 6 bolts and disconnect all the wiring the mounted along it, hopfully they kept the standard tail light plugs in when the tray was put on