@ Aeon: Game info used but NOOOOOOOOOOOO plot spoilers should exist in this post.

Servants are servants forever and there's a chance in every Grail war for any servent of a given class to be summoned, I can't say anything about what might affect these chances but suffice it to say that each war is independant of the others. After a war ends servants go where ever they go and are there until summoned again. This also explains why the Grail can be destroyed in one war then come back ten years later.