Am about to get ahold of a 1981 or 1982 Peugeot UO9, 12 speed, that has a QR front hub but not rear. I see some axle & cone sets on eBay, with axle diameters of 9.5 or 10. Anyone know which? Any other pointers on this conversion? Thank you!

PS: I expect it will have steel rims. I know the tires are 27 inch. I may eventually replace the rims, or the entire wheelset, with new or used 27 inch (or possibly 700?) wheels. But that is down the road a bit.....

this thread probably belongs in the mechanicing forum.

that said, one tip for you. when searching for a qr skewer to employ in the converted hub make sure it is made for 6V as a 5V skewer will be too short.

IIRC, a 1982 U09 I picked up came with rear QR hub. Regardless, you want a 10mm axle.

But given your desire to switch to 700C, it is more cost effective to forgo the QR skewer conversion and seek out a good used set of 700C wheels with all the wanted items.
If you are patient, you can find a pair that would cost only twice what you'd pay for a new axle, cones, and skewer.

Thanks, helpful info as always, and in the future I will study the mechanics forum as well!