If your contractor actually still has a 10 SEER unit, you might as well go that route IF (big IF) the unit he has matches your indoor coil. The product data sheeets for the unit he has lists all the coils it will work with.
As mrbill says, if they arent on that list together, replace the entire system because the new units were never designed or intended or in many cases tested to work with mismatched coils. Now, beware, if your indoor coil fails in the next couple years, you may have difficulty fining a mathing indoor coil the the now new outdoor unit and may end up having to replace the entire system again.
Frankly, to echo mrbill if anyone mismatches a hp, they probably dont know how heat pumps work or understand how the heat is absorbed and rejected. As Dash once said, if they dont know how they work, how will they know when they get them working right?