I wouldn't go that far. Crossbow quarrels, traveling at lower speed, tend to drop a lot more over range than your typical bullet. Crossbowman's going to have to unlearn a bit about adjusting for range. I'd assign a range penalty to a TL4 crossbowman trying to fire a rifle for that reason, at least until he spent an hour or so in a firing range.
Also, crossbows are not famous for emitting loud explosive sounds when they launch their projectiles. We moderns know the "bang" occurs, and it's still a bit of a battle not to flinch. All the crossbowman's training in terms of aim and release will go for naught if he's flinching like a newbie. So, he can figure out which end to point and how to trigger it easily enough, but he's basically a newb with a penalty at long range until he gets some practice in.
5 rounds fired, and 5 minutes, and that crossbowman is using the weapon credibly - not terribly accurately, but able to swap a mag, charge the weapon, and put rounds in the general vicinity of the target.
Also, the crossbow era was also the era of the first grenades in battle... the bang isn't THAT big an issue. And it's not like crossbows are silent. Even a weak one (SCA legal) makes a big twang and is loud enough to be heard inside a steel and foam SCA bascinet helm from a couple meters. Also, cannons were present in many battles, too.
Edit: Thinking about it, the hardest conversion would be the either modern or crossbowman to black powder muzzle loaders or percussion revolvers... Reloading is a huge skill hurdle. Firing? Not a big deal, but reloading in combat is a real problem. I know the steps, and have done it, but it's a pain, and I'd not be competent in a fight, despite being able to strip an AR-15...