Specific criteria to meet to climb to LSA in VFR is outside clouds maintaining vertical and horizontal distance and visibility requirements. I'm sure you already know that.
You can change to IFR from VFR but I dont think ATC will give you IFR clearance till you are above LSALT enroute ( I'll let the ATC people answer that ). But climbing through clouds to get to it under VFR since you cant get IFR yet is out of the question.
In lowering cloud base facing high terrain, I'd do a precautionary search for landing quickly or as SWH said, do a 180 or I'd go for the nearest aerodrome.
Phew, gives me sweaty palms and my adrenaline pumping just thinking about it.
D6