First thing you need to remember is that the cap for the coaches is not at all represented in your salary cap .. dead cap .. etc.
Your teams coaching "cap" is represented on the screen where you offer the coach his contract. Lets use easy numbers:
HC: 3M
OC: 2M
DC: 2M
Pos: 9M ... (9 coaches @ 1M each)
Total 16M
Lets say you want to promote your:
OC to HC ... you offer him 2.5M .. he accepts.
QB coach to OC ... you offer 1.5M .. he accepts.
Your total salary spent on coaches is now 14M ... so now you have 2M to hire a QB coach (instead of 1M).
I hope this helps. the nice thing about coaches is that you can hire/fire at will and there is no dead cap to worry about .. and they are essentially year-to-year as they can be poached each year.