Yes, they can change it (well, this is for the UK anyway, it may be similar).

They CANNOT pay you less money for it, if the position is too different you can demand the "same or similar" job back (so all the new tasks have to go to someone else) else you can walk and don't have to pay them your mat leave money back, because they destroyed your post while you're away.

It's up to you if you want the changed job when you go back - but is this replacement just having to cover your job and their old one? My old job has changed a lot, but that's because my replacement has to do both our jobs in just 20 hours a week (rather than the 40 I had and the 20 she had). It should be more "back to normal" when I return... if there's a job left for me, as there are too many closing down rumours for me to feel secure.