Im thinking of switching to the Global Final Charge coolant product, the van currently has a blue coolant, not sure about which one but I think it's Xerex G48.

I've tried removing the bolts on the bottom of the heads to completely drain the engine but they've not coming out even after using Kroil over night. I want to be super careful not to strip or break off those bolts.

Is there danger in switching to a different coolant without draining the engine from those drain holes on the heads? 

Thx,
Mark