First off, I tried just repairing the internal silicone seam by cutting it out and resealing it with new silicone, leaving the internal seam (silicone sandwiched in between the glass) in place. I let it cure and filled it up with water to see if that would work.
Well, it didn't so I knew the internal seam was more compromised than I thought. You can see in the second picture about where the leak was as the tank drained down to that spot.
Next step, complete front glass pane removal!
I removed all of the silicone with a lot of elbow grease, a flat razor, and some of this magic stuff I found at Home Depot.
I went ahead and ordered two fresh tubes of silicone for the reseal as I was going to generously lay it on this time around and not be so stingy with the silicone. I used Dow Corning 732 clear silicone on OffRoadDodge's recommendation.
While awaiting the silicone to be delivered, I went ahead and moved my old 125-gallon out of the way. I drained the water and left about 4" of water and sand in the tank (plus the fish), emptied the sump, put all the coral in the 180-gallon temporarily, and then used a whole lot of leverage and a whole lot of positioning against the wall to push it to the side so that I can put the new 215-gallon in its place.
Corals all in the temp tank:
My precious cargo all in one place!
Other than a little phosphate spike, all was well and tank is pretty much back to its normal self.
After the move and carefully placing all the corals back: