Systems and methods for unsticking a tubular string in a subterranean well that is a stuck pipe include securing a circulation sub to an uphole end of the tubular string outside of the subterranean well. The buoyancy of the tubular string is increased by delivering a replacement fluid through the circulation sub and into a bore of the tubular string. The replacement fluid displaces an operating fluid located within the bore of the tubular string. A density of the replacement fluid is less than a density of the operating fluid. Conventional operations for unsticking the tubular string can be performed. The replacement fluid in the bore of the tubular string can be replaced with the operating fluid, and the replacement fluid can be delivered back out of the subterranean well.