[Don Wright]

Assuming the arm is considered a "mechanism" consisting of "parts", accodring to R15, you can make upgrade "parts" during the fix-it window after the last regional before the championship event. The upgraded "parts" when assembled into the arm "mechanism" made it lighter and stronger. Even though the second half of R15 doesn't explicitly say you can use the upgraded parts to fix the mechanism, I just think it doesn't because it was a cut and paste from the rule for the fix-it-window after any regional and they added the upgraded parts part before it for the championship event.

This is legal.

R15: During the “FIX-IT WINDOW” following the last Regional Event a team attends (i.e. prior to attending
the FIRST Championship): Teams may manufacture SPARE, REPLACEMENT and UPGRADE PARTS
for their robot at their home facility (not at the competition site). At the conclusion of a regional
competition event, teams may take a limited amount of broken or malfunctioning COMPONENTS or
MECHANISMS back to their home facility to make SPARE or REPLACEMENT PARTS. Teams may
manufacture and/or repair all the parts they want, but the amount of parts they can bring to the
competition event is limited (as specified in Rule <R24>).