This is prolly going to sound like a dumb question - but will a Gen4 9mm upper function OK on a Gen3 G31 lower? I found one on Crook Broker at a semi-reasonable price and would like to get it. Of course, the seller says it will work but I wanted to ask here first.

I have a NIB G31C that I picked up in Germany around 20 years ago and have never fired it. I asked the owner of an LGS, here, what he thought it would be worth on consignment. He was nice about it - but the bottom line is full-size Glocks are a dime a dozen. I prolly wouldn't get $300 out of it. So, I decided to put the lower to work as a G17 and put a TLR-6 on it.

Yes, I know, you don't have to tell me: what the hell was I thinking getting a G31C? ... This was back in the days of getting a handgun here and there at the Rod & Gun Club in Germany. You couldn't just walk in and say, I want an XYZ. You took what they had in stock and vergessen about getting something from the States. This one came from the Darmstadt Club before they closed it and the kaserne (as far as I know) down in the early 2000's.

As I understand this you cannot. I researched it for another reason but the answer seemed pretty consistent. I did not try it. The recoil spring difference leads to issues.
But since there are about a zillion G3 17 slides available, why not just go that way?

Gen3 and Gen4 frames and slides are not interchangeable.

EDIT: except for the subcompacts (G26, G27 and G33). I forgot about those.

Thanks Gents - I'm glad I asked. I guess I'll just get a LWD 9mm conversion barrel. .357 Sig, at least in Glocks, seems pretty dead around here - and six-feet under with a ported barrel like on my 31C. All is not lost - I remember it being around $350-360-ish from the R&G Club years ago so I didn't have to sell the farm to buy it...

G3 uppers are everywhere. A conversion barrel works mostly for sure, but I'd never use one for serious work. If that's your goal just get a complete upper and be done with it.