Switching Tier 4 dependants to Tier 2 dependants

[Halleberry]

Hi, I am switching to a tier 2 visa in a couple of months although my current TIER 4 visa expires in 4 months time. My family are all dependent on my TIER 4 visa. My questions are these:
1. My employer is willing to show evidence of maintenance funds for my family, do we need to apply at the same time, or can i apply separately for them with this evidence from my employer?
2. If i need to apply separately for them after my own TIER 2 visa comes out, how long after i have switched my visa can i apply for my dependents, without them having issues of overstaying in the country. Thanks