It may be ok and it might not be ok, ask your structural engineer.
The collar ties basically stop the tops of the walls from spreading apart. The only way for the roof to fall in (short of the rafters breaking) is for the rafters to push the walls out. In a true cathedral ceiling you don't need collar ties. This is because they have a "ridge beam" that holds up the roof load and is posted up on either end. I have seen cases where the collar ties were moved up higher than the wall top plates, but this was always done with a structural engineers plan. If they are raised significantly then you might need plywood or metal gussets to reinforce the rafter/collar tie joint. Also you need extra fastening, and maybe metal straps, of the rafter/wall top plate connection. Usually, it is not a big deal to raise them one or two feet above the top of the wall if the connections are strong. Is it ok or isn't it? Only your structural engineer knows for sure.