I was faced with the same dillema, and was advised by all on FT that changing use order of coupons requires a re-issue. In fact the conditions on the tickets clearly state that and most airlines have that condition as part of their Conditions of Carriage.

The consensus was that one might belucky if a check in agent didn't notice or didn't know and let you fly them out of order but one shouldn't take that chance.

I didn't. I re-issued at LAX QF, and managed to argue them out of the $75 by 'pleading ignorance' with the agent that this "wasn't really a re-routing, just a different order of doing the same route"!!

It worked. But I think I lucked out