WAR is a counting stat, so accounting for in season roster changes is as simple as summing up each individual's contributions. i. e. playing time is already factored in.
As the name implies, Wins above replacement describes performance above that of an arbitrary baseline skill level (approximately that of a 25th man or a "quad A" lifer). Last I checked, a team full of replacement level players would be expected to win about 43 games, therefore team wins can be predicted by 43 + positional war + pitcher war.
*note however that the stats won't match team wins exactly on account of the inherent variation accrued over the course of the season