How should I raise the subfloor in one room to match another?
I would always remove floating laminate before installing a new flooring. For one thing, it's probably not an approved substrate for your new product--you'll likely void the warranty by leaving it in. For another, you open yourself up to unwanted movement and noise.
So yes, remove the laminate and its underlayment (felt, foam) if present. Then assess the height difference. If it's about 3/4&quot; like you describe, I'd overlay and screw down 3/4&quot; tongue-and-groove OSB. It's about as cheap as you'll find for 3/4&quot; material.
If you then have a small height discrepancy you can skim some floor leveler across that transition, feathering it out with a straight trowel for at least 12-16&quot;.