Replace plugs as they were taken out?

1995 Buick Lesabre, with 110K miles on it. Clean engine.

I had Bosch Plus 2 (+2) plugs in it, ran very good, extremely quick response, but gas mileage has been terrible for some time, even before these plugs.

I decided to try some super-ugh plain AC Delco plugs, in hopes a normal spark might allow better mpg. But response has been much less, and harder to start car, have to crank it now. So I'm going to give up on the plain plugs and reinstall the Bosch.

But when I first removed the Bosch plugs, the front side of V6 had slight carbon fouling on plugs, while the rear plugs looked almost brand new.

My question is, should i replace the slightly fouled plugs on front side again, or put them in rear, where they won't foul any more, with like new plugs on front. Possibly to get better response.

Putting slightly fouled plugs back in front, they'll keep fouling a tiny bit more, while like new plugs in back, stay like new.

If I put like new plugs on front side, they'll foul tiny bit, and eventually get to where they match the fouled plugs in back.

When I say slightly fouled, means tiny bit of carbon burning on plug tips.

PS----took car to family mechanic, to get tuneup. Figured he could set computer to make sure it was working properly. But he charged me $350 to change spark plug wires, that's All. He told me the computer will adjust car, depending upon how I drive. And I'd just changed the plug wires myself, few months earlier!!

I'm getting about 15 mpg, even on highway.