Changing the ticket will always revalidate all of it.
Your new itinerary violates the 7-day minimum stay provision of the original fare. Using fares in effect at ticketing, this would reprice to PHX80NPJ outbound (USD 1380.00 add/collect) and AHX70NP1 (USD 245.00 add/collect). In theory the GUC should need to re-clear, and not sure if there is space (Z0 alone, Z3 married from PDX).