My house has a 200 amp service with a (circa mid 1980s) GE TM2020R Mod1 panel where all 20 spaces are occupied with THQP breakers. In order to provide room for expansion and to allow for GFCI and AFCI breakers, I was planning on adding an additional outdoor 200A loadcenter, fed by subfeed lugs, adjacent to the existing loadcenter. The general idea is outlined by ThreePhaseEel in "The Streamlined Way" section of this post, although both of my panels will be located within (much) less than 24 inches conduit run of each other outdoors. The conduit running between the panels will be sized for the largest side knockout on the panels, which I believe is 2.5 inches.
Several circuits will need to be moved to the new panel that currently enter the existing panel through knockouts on the back of that panel. It would be considerably easier to leave these circuits entering where there currently do, and then run the conductors through the conduit connecting the panels, than to move them entirely to the new panel. Given that derating is not an issue due to the short distance between the panels and 60 percent conduit fill is observed, is there any prohibition in the 2017 NEC that would prevent doing this?
Also, given that the cost difference for such short wires is small and conduit fill is not an issue, is there any reason to prefer CU 3/0 or Al 250mcm conductors for the feeders between the panels?