I think that replacing is not the right thing to do.
Just make a Firearms branch, with two or three weapons:
A crappy handcannon, short range, inaccurate as hell, and crap damage.
A simple Arquebus, moderate damage, and moderate reloading speed. Also moderate in accuracy. A balanced killing tool.
A complicated Sniper Arquebus, Deals high damage, Takes AGES to reload, but it can go a bit further than the simple Arquebus.
This one should have a bit better accuracy than the simple Arquebus, but it should still miss sometimes, like the Sniper Crossbow.
Also, the costs of these weapons should be pretty high, so plate gunners won't be a problem.
Another option is to balance the guns with the crossbows, so the guns will be only new skins, within a new branch.
Perhaps lower the reloading speed than most crossbows, and add a BIT more damage, to make them different, but it should leave crossbows a viable weapon.
What do you think?