The BUMP program now excludes America West, Frontier, and ATA (and of course those carriers that NW has no ticketing agreement with such as WN, B6, SY, NK and the like).

In your case, mdb, it becomes an agent decision. If the DTW-PHX flight is full/overbooked and there is space on the NW DTW-SAN flight, it might be a win/win for NW to accept the HP coupon in combination NW DTW-PHX coupon. NW would receive the revenue for both coupons.