You could do this job yourself if you are happy to do so and are competent. I reckon most chanders will fit the kit for you, I know some do it for free.
Firstly contact the chandlers to be sure they have the kit for your particular make and model (there are different systems in different jackets.... halkey roberts ....hammar hydrostatic etc.) the kits are different for auto and manual inflate.
The mechanism should be fine and you might only need the cylinder. You could fire the jacket before replacing the cylinder just to see it work

. Make sure the cylinder is properly seated when screwed in.
No harm to inspect the bladder and leave it inflated overnight to be sure it stays inflated.
All the manufacturers tend to have re-arming instrucions on their sites:
Well worth a read!
http://www.cmhammar.com/images/uploads/M1A1-gul.pdfhttp://www.parmaris.com/customerinformation.htmlhttp://www.baltic.se/_pdf/_eng/info_inflatable.pdfhttp://www.mullion.org/faq/lifejackets