An MCA upgrade modifies an existing site to add capacity under the carrier's program requirements: new spectrum bands, more radios per sector, an additional tenant, or a heavier antenna load, usually with some combination of structural reinforcement, new mounts, new transmission lines, and updated ground equipment.
Adding capacity to an existing tower is generally cheaper and faster than building a new tower, especially with carriers rolling out new bands (C-band, mid-band 5G, mmWave) on infrastructure that's already in the ground.