No need to redesign the form. Both releases use the same Oracle forms and reports version.
Just compile it, place it on the server and generate the fmx.
Of course you must test it to ensure it works correctly.
If this is a customisation to a standard form then you should of course check that the form hasn't changed between 12.1 and 12.2 - if it has then you'll need to reapply your customisation atop the 12.2 version.