Switching From Tier 4 dependant to Tier 2 Dependant

I am currently in the U.K on a Tier 4 VISA and my wife is a dependant. I have now gotten a job and I will be switching to tier 2 however I am unable to apply at the same time with my wife. My wife's dependant VISA is still valid for about 18months. I want to know if my wife can continue using are tier 4 dependant VISA until she is able to apply or will her tier 4 dependant VISA be cancelled once I switch meaning we have to switch at the same time



Thanks