Hi Barry,
The 55020 is quite an old unit, but provided it is working OK I don't see a problem; when you say "is it easy to change", do you mean to upgrade to (say) a sound decoder, or just to replace it with a more modern type? Either way, as the loco is already decoder-fitted that means it is a pretty simple job to change it up to something like a Massoth XL (or XLS if you want sound), should be just a matter of unplugging the old and plugging in the new. If it was an older loco that was purely analogue, THAT's when decoder fitting can get a little trickier (though even that is pretty straightforward once you've figured out what you're doing and got over the fear of dismantling gearboxes and the like!).
If the price is right, then I'd go for it!
Jon