As Jobst Brant says, a shop doing the build will normally advise replacing the spokes because they know you will blame their workmanship if you start breaking spokes soon after. (And it is a little faster, but wasteful of perfectly good spokes, to just cut the old spokes out of the old rim and start fresh. Depends who's paying I guess. One man's expense is another man's revenue.) But if you haven't been breaking spokes, there is no reason not to reuse them exactly as Prathman says -- I've done this many times without problems.

Now, if you find the nipples are seized to the spokes or to the rim sockets, it is going to take some time (= $$ even if it's your own leisure time) to get them transferred into the new rim. You might end up rounding off some nipples or even breaking a spoke or two from torsion. When a rusted spoke breaks, it flings off rust particles right into your eyes -- wear safety glasses. The free end of the broken spoke doesn't usually shoot out of the rim like a bolt from a crossbow (because you're grabbing it with the spoke wrench), but you should still avoid looking "down the barrel" so to speak.

However, what I bet you find is that the ERD of new rims are just enough smaller than old standbys like MA-2s that you won't be able to re-use the old spokes. (If you are upgrading modern el cheapo rims to Open Pros you might be in luck, but since this is a C&V forum I was guessing that you were replacing worn-out vintage rims.)