Well, bad news.....I believe your only choice is to explode the object down into a 3D solid and create the mvpart again. Once you create a part via partconvert you cannot modify it. While on the surface it sounds good to use part convert in reality you should always just create a catalog based part for this very reason. The steps involved are almost identical and only takes a couple more minutes.
The good news for you is that you still have the 3D geometry so you will not need to recreate the geometry of the part. Once you explode it down to a 3D solid it should only take a couple minutes to redefine the part.
p.s. make sure you dont' explode the part down to far into a surface. MvParts should always be 3D solids and not surfaces.
Hope this helps.
