you can change the routing but the rules need to be complied with. Otherwise, there can be a refare from point of origin plus change fee. If open jaw are allowed with the open segment at origin in the same country and no restriction for the segment at destination, and the open segment is shorter than the longest flown fare component, for the new fare component (even if the old is strict return trip) the following change should work:
CDG HEL CDG changed into CDG HEL // MOW CDG NCE. Of course, one needs to pay the difference between CDG MOW and CDG HEL. If NCE CDG MOW is cheaper than the paid CDG HEL, usually no refund is due on restricted tickets (with some exceptions).