On Friday Josh and I went back to Arch - Up the Center.
We replaced four bolts. We did not have time to try to lead the route. For those who read these types of things, here are the details:
Mister Mucci moved up onto the big hook that is just below the first existing hole (the one below the old fixed pin, shown in the photo posted above). From this hook Josh measured the depth of the existing hole. It was about 1 1/4 inch to 1 1/2 inch deep and 3/8 inch diameter. No bat-hook hole would have worked here, and no one would drill a bat-hook hole that deep (also confirming that this was a bolt hole is the length of the fourth bolt, which was still in place up on the lip of the roof, and which we pulled and replaced; this was a 1 1/2 inch long Star Dryvin).
Not having bolts that short, Josh drilled the existing hole deeper and placed a 3/8 inch rawl with a very nicely camouflaged hanger.

He then tried to move up on the old fixed piton in order to find and then replace the bolt that had been in the second hole. The piton shifted half an inch. Josh wasn't willing to stand up on this piton to find and then spend 30 minutes replacing the next bolt. Since Josh was obviously being a pansy, I went up and tried the fixed pin. It shifted again. Attempts to re-drive it sounded hollow and totally unconvincing. I was therefore also unwilling to stand on this piton and find and replace the next bolt.
Our further attempts to replace resulted in extreme shenanigans. However, eventually, we got it done.
Since neither of us was willing to stand on an obviously crappy fixed pin, we decided to get to the second bolt hole the way Phil and I had - by traversing in from the right. But both Josh and I had done this before and we knew it was a pain in the ass. And I'd brought only about 40% of my rack. We soon realized that this long traverse was more than we were willing to do without adequate pro.
So we decided to at least replace the fourth bolt while we were there. To access this I climbed the tree. This is a lot harder than it sounds. Josh and I used a series of rope throws over branches to protect what might have been the damp equivalent of 5.9 climbing. Eventually I got high enough to throw the rope over the huge branch closest to the route. Josh fixed the rope and I rapped. He then jumarred the rope, clipped the existing bolt and proceeded to spend 40 minutes hanging while he placed a 3/8 inch Rawl in good rock six inches to the right of the existing "at-the-lip" bolt. We were a little surprised how short the existing Star Dryvin bolt was (1 1/2 inches), and at the (poor) quality of the rock it was in.

We'd replaced the first and fourth bolts. How to access the middle of the route?
The answer came from two Santa Cruz climbers, John and Kathy. They were nearby and had a nice, long, cheater stick with them.
Since Josh was already in place, he got to continue (he did all the work, I can't believe how easy I had it). From his position at the last bolt, Josh used the cheater stick to clip first one and then the second of the two fixed pins I'd left in the crack under the roof. Using these clips, he lowered down and in toward the main face. He clipped another fixed pin that I'd left in the crack where the roof and face meet, but that pulled out with minimal weight. So, Josh used the cheater stick to shove a cam up into the crack. He was then able to stand on this in his aiders (so by this point his rope went from a tied off tree at the ground, over an upper tree limb, down to and then under the roof and to the main face - the swing-out if things had failed would have been safe but quite bottom clenching).

Once in place Josh saw not one, but two more now-unfilled bolt holes (I had suspected a third hole as I described in a post above). Upon inspection each turned out to be 3/8 inch diameter and 1 1/2 inches deep.
Working from the top down, Josh drilled the upper hole (the third bolt/hole in order from the ground upward) deeper. He placed a Rawl in this hole. The hole below this (the second one up from the ground) turned out to be a problem. Once Josh put the drill in the hole and hit it with the hammer, we both realized that the whole area of rock was crap; it was totally hollow.
What to do?
It seems like a terrible idea to replace a bolt into a hole that one knows to be bad. We agreed on this. So, we had to either not replace the bolt, or put it in a different (newly drilled) hole. Having come to "restore" the route, we opted for the latter. Josh drilled in bullet rock about 14 inches left and slightly lower that where the existing hole was (and still is - unfilled).
We worried slightly that this (now third) bolt location would allow people with a long reach to skip the fixed pin by reaching to this second bolt while standing high in aiders on the first bolt. But then we realized that I had done just that while standing on the first bolt (in aiders). I'd reached up and put my finger on the existing second bolt hole. So, if we'd put that second bolt in the same hole, the result would have been the same.
We then retrieved our rope and gear. While doing this we experienced an ominous event: The first of the two pins I'd fixed into the roof crack, a pin I'd stood on with all my weight (and Josh too!), fell out as Josh flipped the rope around to clear it and the gear. Oh boy. I can't wait to go back up and replace and stand on that pin again.
Thanks, Josh for all the fun and for doing all the work.