From the 2020 Game and Season Manual. Obviously the 2021 rules are very different, and we have yet to see how this will apply in future years.
R14. FABRICATED ITEMS created before Kickoff are not permitted. Exceptions are:
E. COTS items with any of the following modifications:
i. Non-functional decoration or labeling
ii. Assembly of COTS items per manufacturer specs, unless the result constitutes a MAJOR MECHANISM as defined in I1
iii. Work that could be reasonably accomplished in fewer than 30 minutes with the use of handheld tools (e.g. drilling a small number of holes in a COTS part)
A heavily modified AM14U chassis (one that is not exempt under E. iii. above) would violate R14. For a minimally modified or unmodified COTS chassis, assembled “per manufacturer specs,” I1 becomes the sticky part. MAJOR MECHANISMS are a big gray area. As I read it, though, unless your chassis has everything it needs to drive on its own, it is not a MAJOR MECHISM.
I1. It’s your team’s ROBOT. The ROBOT and its MAJOR MECHANISMS must be built by the FIRST Robotics Competition team.
A MAJOR MECHANISM is a group of COMPONENTS and/or MECHANISMS assembled together to address at least one (1) game challenge: robot movement, game piece control, field element manipulation, or performance of a scorable task without the assistance of another ROBOT.
Something I just thought of, that maybe an RI can jump in on: Would work done in accordance with R14 E. iii. (e.g. shortening frame members) nullify the COTS status and likewise a potential exemption under E. ii.?