Is there much to making this change? I'm looking at an older A-square rifle, from the nineties before the company changed the action to cock on opening. If it's not a big deal to make the switch, I may buy it.

Dayton Traister sells the kit pretty cheap about 25-30$ if I recall. They also sell one for 96 Mausers which I've purchased several of.
The conversion is not complex depending on what you have for tools. You can go to their website and look at the install instructions and see if it's what you want to tackle.
Don't get the Dayton Traister trigger, get a Timney or the like or use existing but I know 2 Gunsmiths who commented on the difficulty of their trigger.

My only exoerience is the one from Numerich.
The firing pin drop is so short that youneed a heavy spring. The heavy spring makes opening the
action harder. It seems to be a fix of a non
problem. I junked mine.

Ed Lapour makes one, you won't fine a nicer one.
Don

It's a solution to a non existing problem. Cock on closing was designed for a reason by the British; it makes initial opening and extraction easier so the forces of initial extraction and cocking are spread out over two movements instead of one. In combat, that method is faster especially with dirty or sticky ammo. I can shoot my Enfields and 1917s much faster than a Mauser or 03. Practice with it.