I don't really know but my guess would be don't try it. While Yamaha makes all their own keybeds in house my guess is since the P120 is two years older than the P255 there's probably been some mods made since then that wouldn't allow a direct swap.
However, Yamaha had an outstanding recall on P120's with sticky keys. Apparently it's a known problem and they offered a free parts only replacement of the keybed on units exhibiting this problem.
The Yamaha P-120 is a portable electronic piano. The 88-key keyboard is action-weighted, imitating the feel of a real piano. It includes several sample keyboard sounds, such as harpsichord, clavichord, vibraphone, guitar and more. Basic sequencing and editing are built-in. Excellent feel to the keys, good tone, easily accessed ports on side (not back). Problems with keys sticking have been reported.
The sticky key problem is caused by the plastic used in the manufacture of the keys for the P120 reacting with the grease used to form an adhesive like substance. This has been recognised by Yamaha. Yamaha will replace the key-bed (part cost only) free of charge if sticky keys are experienced. In 2012 this changed to free part only and the end user pays the labor cost. On December 31, 2013, the replacement keybed program will come to an end and there will be no more free parts offered in the USA.
While the recall may be officially over if you are experiencing this problem I would try my damnedest to collect on this recall.
Possible suggestions include filing a report with the local BBB as well as the local consumer advocates in the media if they don't offer satisfaction in this matter.
Considering they knowingly sold a defective product to you, even small claims court is an option. Chances are they'd rather settle than show up in that case. 3 hours of a lawyer's time is way more costly than a simple settlement in this case.
If you open it up you blow any chance of a warranty repair out of the water, but if it's still got the factory seals on it then Bob's yer Uncle and you have a good shot at making them fix it properly.