Tank Talk

You can, yes.
Understand that it could be a rather large mess depending on the 'planted' substrate you choose to use. 


The easiest way would be to move 1/2 of the current substrate to the opposite side of the tank, then add your new 'planted' substrate.  Then do the other end the same way, placing your old substrate on top of your newly laid substrate.
When complete, simply spread the old substrate over the new for best results.


You can, however simply mix the two together.  You'll still see a benefit to your plants growth, yet not as beneficial as you would if the grow media was on the bottom for the roots.

Good luck!