It'd probably involve swapping their level of Warrior 1 to Fighter (Trench Fighter) 1. CR should go from .5 to 1.
If you want to do it without changing their class, the good rule to remember is that an encounter of a certain CR is expected to have about a certain amount of wealth. There is a table for this. If you reduce the wealth of the encounter, it might lower the CR (but some creatures don't use their wealth - a lion with a pile of coin in its den is not harder to kill than a lion with no coin, so this rule only applies to creatures that SHOULD use their wealth, which includes common soldiers). Likewise, for these same creatures, increasing their wealth should increase the CR.
How much to increase it is up to you, the GM. There are no specific rules for it, so it is a bit of an art form.
Me, I would say if they have just a little bit of extra wealth, maybe even up to 2x their expected amount, I wouldn't raise the CR. If they have a lot, at least 2x, I might bump the CR up to the next CR (so a CR 3 becomes a CR4, but a CR 1/2 only becomes a CR 1). If it's more than whatever I would call "a lot" then it gets into the realm of being a bit ridiculous - but if I did that, I would just have to start making judgment calls (a level 3 fighter is CR 2, but give him +5 full plate, +5 shield, a +5 vorpal sword, and a bunch of magical gear to resist enemy spellcasters, and this guy is a lot harder to kill than just CR 3 or CR 4, but how much is hard to evaluate because he's so ridiculous).
Note that the above assumes the wealth is in the form of gear that can be used to win the encounter. A bag of gold doesn't change the guy's CR, but the same value of arms, armor, or magic items certainly will, if the guy can use them (having several suits of +1 armor doesn't change the CR because the guy can only wear one).
A CR 1/2 soldier is expected to have about 130gp worth of gear. Giving him a musket (1,500gp) . Assuming they can use these muskets, they definitely should increase their CR. Heck, if your PCs engage these musket men outdoors in good visibility, this encounter could end while they're still a couple hundred feet apart (a few range penalties, offset by touch attacks, could be devastating).
I would probably treat this as a minimum of 1 CR increase, maybe 2 (so from CR 1/2 to CR 2. Unless maybe there is an easy obvious way to neutralize the muskets, like catching them in a narrow tunnel and forcing them into melee without ever using the muskets).