I just replaced the PR2s at 14,000 miles with a PR3 rear, and a PR2 front. The old front PR2 still was not near the wear bars and probably had another 5,000 miles. The rear was done. So far, I have done a break in ride from Phoenix to Payson, to the rim, to Camp Verde, and back home on the interstate. Tires are working well together. Lots of sweepers and a few 20MPH curves. Great fun.
OK, I know I'm probably stripping line off your Zebco, but if you are OK with running a mismatched set of tires, why replace the front when it has 5k miles left on it?
Purchased 4/16 with 31424 miles on the Bike ..
New Tires have .. Front 5/32 .. Rear 8/32 and 7/32 off center.
Today 5/21 .. Bike has 32816 which equals 1392 miles on tires .. and they read Front 4/32 .. Rear 6.5/32 and 5.5/32 off center. (btw running 40 psi)
40 psi is too low for the back, especially if you want to get 10k miles. You'll need to run at least 42 psi rear (and also not be too aggressive on the sides) to get that kind of mileage.
And yes, 2 psi makes a big difference in the tire heat, and therefore wear.