The ammo I carry everyday isn't under conditions any different than the ammo I have stored at home, in any significant way I can think of. It's certainly as dry, and probably sees less temperature swing than my stored ammo. In any case, I've got nothing to suggest temperature swings from 0F to 90F are ever going to have any impact on ammo anyway, and that's generally the widest range my ammo or powder will ever see...with some rare exceptions.
I see no reason at all to ever change the ammo in my carry guns...it's no older or newer than the ammo I'd replace it with, and I'm pretty darn sure I could chamber any of it about a zillion times and it would measure exactly what it does right now. It certainly wouldn't be set
back from chambering.
Anyway..I certainly don't care if others choose to change out their ammo from time to time...but it's just not a concern of mine. I choose to hand inspect each round (usually) for case mouth burrs, extractor burrs or damage on the rim, poor primer seating/squashing and so on. I clean each round by hand, and load 'em in a magazine that's been disassembled and cleaned and 'feels good'. The carry ammo all comes out of the mag when I practice at the range, and it all goes back into the mag when I'm done shooting and ready to reholster to leave the range.
Under any conditions I've ever carried or kept ammo, I'd consider 'old' or possibly less reliable to be no less than ~25 years old for factory ammo or any ammo I've hand loaded in clean cases. For folks who somehow manage to sweat all over their ammo, I suppose there could be greater concern.
I happen to live in a bone dry high plains desert environment, and I know this makes a big difference--I was born and raised in Michigan and things were a lot different there.
Was JHP ammo a specific concern in this context, or was it just an assumption that carry ammo = JHP? I didn't get why the OP specifically mentioned JHP.