If it's the modern type of baluster arrangement, Eth, then it's a simple job to replace them. The spindles fit into a slot on the underside of the top rail, and into another slot in the bottom rail.
Small pieces of timber are tacked in between each spindle to hold them in place. No need to touch the newell or the top rail.
A different story if they are older. Over thirty or forty years ago, they were often mortice & tenonned into top and bottom. The top rail would have to come off to dismantle. Often a messy and difficult job.
You could take out 25 modern spindles, and replace them in only a couple of hours. Then, there'd be a certain amount of time to fill and re-stain.
Replacing an older, traditional set could take a day maybe. Impossible to be sure without looking. It'll depend on how they're all put together.
Depending on where you live, and who you employ, hourly rates can be anything from £12 to £30 for this kind of work.