I am not an APX expert by any means, but I am pretty sure there's not any such option in the APX. At least there's no such option in my APX8000.
I am guessing this is due to the timeline for the APX product line in relation to the chronology of the FCC rebanding orders and execution of those orders across the LMR industry.
In searching the "Help" files in CPS there is some mention of Old/New selections in relation to Primary and Secondary Band Plans where either of those bands were 700 or 800 MHz. I think that maximum TX power levels are pretty much fixed in firmware in accordance with the FCC rules since the completion of rebanding.
Hopefully, someone will chime in with more factual information that I have been able to provide.