Dude dont worry about the shortcut as it can throw you. Learn from the main menus then create your own.

Anyway the answer to your problem is very simpe. To convert the poly to a sub D go to the Modify menu>Convert>Poly to SubD.

Okay, when converting back to the base level mesh, again go to the Modify menu>Convert>SubDs to Poly, but to get it back to the base mesh, hit the vertices radio button and put the Level setting on the left to Zero. Sorted.

Jay