Depending on the hilt and all the different variables, it may he cheaper to buy a new hilt with sound down the road. Do the math.
You'll need a pommel that has vents to let the sound out. Will your current saber have that?
You may need to upgrade/downgrade the electronics. Are you buying it spec'd out to accommodate the sound version you want?
You'll have to pay for shipping both ways. How much is that?
You'll need to buy the sound board and speaker. How much is that?
You'll have to pay for the labor. It's usually at least $50 any time they touch the saber. It might be more.
It's definitely doable, but some have found it's just cheaper and easier to get a brand new hilt, if that hilt is on the lower end of the price spectrum.
I was already planning to swap out the pommel, so that wasn't a big deal. I am starting with stunt sabers until I figure out exactly what I like. Since I am probably buying PO for my first saber, it may be better to use a buckpuck setup for that anyway, to regulate the color more. Right now, I don't have a definite timeline, but do need my first saber in hand by September, so I am just thinking about down the road for now.