Tier 4 to Tier 2 (gen)-with or without dependents?

[jmack]

Context: I am interviewing for a research post soon which would require a Tier 4 student to Tier 2 general switch (I am eligible since I have completed more than 12 mos of my PhD). I have a spouse and child on Tier 4 dependent visas (good to 2016) and a second child recently born who does not yet have a visa. We all live in the UK presently with no plans to leave the country in the foreseeable future.

Question: May I initially apply for a tier 2 general visa separate from my dependents and, if granted, apply for them in a few months from within the UK? Reason: I will likely have to use the premium visa service to start my job on time (an extra £400 per applicant!), and I would like to save on the cost of the dependents' visas.

My concern is that a tier 4 dependent visa will be voided if the main tier 4 visa holder (me) switches to Tier 2.