Typically, normal promotions are fine and do not require any special procedings. But if u are moving way higher like from a "junior developer" to "VP of technology" there is trouble in ur GC as ur job description would have changed..
AC-21's basic tenet is change of "employer" while having same or similar "job description". If on promotion ur job description can remain the same (in case u get RFE, ur employer is willing to give a same job description letter with your new title) - u don't need to do anything as u are not changing ur employer. If ur employer is not willing to give a same or similar job description letter in case of an RFE - AC-21 would not help anyways.
Best bet is to convince employer for helping u out with similar job description letter in case of RFE - most employers I know are willing to assist with it..
Goodluck!
-ab
