Can I use my old shower pan instead of a mortar base layer?
The existing drain assembly terminates at the height of the current shower pan.  Your proposal would require a drain extension to the new, higher shower pan.  This would require a smaller diameter fitting, inside the old drain, in order to extend the height of the new drain.  This would be practically difficult, if at all possible, because of the short lengths of pipe involved unless you're creating a "step-up" shower.  By the way, such a fitting violates code by reducing the diameter of the waste pipe (even though just at the shower pan). 

To avoid the above, the old shower pan would have to be excavated, at the drain, to allow the old drain assembly to be removed and a larger fitting attached; into which the new drain would be fitted. 

Note: raising the height of the shower pan will result in faucet handles and shower heads being lower than they are now.