Hi so this is gonna sound really complicated and weird:

I need a script where after a specific round the player can go up to a certain trigger (or object) and upgrade their gun.

This sounds really strange but it'll look good.

so round activated pap

Kinda but its basicly a second PaP but its not the actual PaP machine model.

In the trigger think loop add if(level.round_number < X) continue;

Its gotta be separate from the original PaP. The original is still in, BUT there is a alternative in spawn that activates at round 15.

RoundPap(){
	cost = 5000;
	papRound = 10;//change for the round you can pap with this trigger
	papTrig = getent("paptrig","targetname");//change paptrig to your targetname kvp
	papTrig setCursorHint("HINT_NOICON");
	if(isdefined(papTrig.zombie_cost)) cost = papTrig.zombie_cost;
 
	while(1){
		level waittill ( "between_round_over" );
		if(level.round_number>papRound){
			papTrig sethintstring("Press &&1 to upgrade [Cost: " + cost + "]");
			papTrig thread AllowPapTrig();
			break;
		}	
	}
}
AllowPapTrig(){
        iprintlnbold("This trig is now available for pap");//comment out after testing complete if desired
	while(1){
		self waittill("trigger",player);
		current_weapon = player getCurrentWeapon();
		if( !IsDefined( level.zombie_include_weapons[current_weapon] ) || !IsDefined( level.zombie_include_weapons[current_weapon + "_upgraded"] ) )
		{
			player iprintlnbold("The " + current_weapon + " is not upgradeable");//comment out after testing if desired
			continue;
		}
		player TakeWeapon(current_weapon);
		wait(.2);
		player GiveWeapon(current_weapon+ "_upgraded");
		player SwitchToWeapon(current_weapon+ "_upgraded");
		wait(.5);
	}
}

You could try something like this:

RoundPap(){
	cost = 5000;
	papRound = 10;//change for the round you can pap with this trigger
	papTrig = getent("paptrig","targetname");//change paptrig to your targetname kvp
	papTrig setCursorHint("HINT_NOICON");
	if(isdefined(papTrig.zombie_cost)) cost = papTrig.zombie_cost;
 
	while(1){
		level waittill ( "between_round_over" );
		if(level.round_number>papRound){
			pap sethintstring("Press &&1 to upgrade [Cost: " + cost + "]");
			papTrig thread AllowPapTrig();
			break;
		}	
	}
}
AllowPapTrig(){
	while(1){
		self waittill("trigger",player);
		current_weapon = player getCurrentWeapon();
		if( !IsDefined( level.zombie_include_weapons[current_weapon] ) || !IsDefined( level.zombie_include_weapons[current_weapon + "_upgraded"] ) )
		{
			player iprintlnbold("The " + current_weapon + " is not upgradeable");//comment out after testing if desired
			continue;
		}
		player TakeWeapon(current_weapon);
		wait(.2);
		player GiveWeapon(current_weapon+ "_upgraded");
		player SwitchToWeapon(current_weapon+ "_upgraded");
		wait(.5);
	}
}

Not tested for syntax or anything, just typed up on lunch away from gaming pc. Nothing fancy.
You would add this to your mapname.gsc and add thread RoundPap(); up in the function thread section

my bad, should be papTrig sethintstring.... Updated in my op, and added iprintlnbold to say it is ready for pap for testing