A few years back every license required a full exam every time. Now it depends on what license, what you already have, and what the evaluator feels like doing. There appears to be a lot of variation between evaluators.
I know a third mate that recently upgraded to master 1600 and master of towing. He did not have any unlimited tonnage seatime to upgrade to second mate first and qualify for the cross over exam to master 1600. They made him take every module for both exams, except rules of the road, even though the master of towing modules were just much simplified and shorter versions of the master 1600 modules. He took 11 modules in three days. Silly, but no problem.
When upgrading above 200 tons, the only big differences in the exams are much higher levels of T-Nav and C-Nav. Going from 200 tons up to 500 tons or more is not a lateral move. Mate 500, mate 1600, mate OSV, third mate, and second mate, are as I understand it all the same exam.
The differences in the exams between third mate, master 500, and master 1600 are trivial. They could just add the little bit of management level material to the third mate exam, and make all of it the same exam. Its the government, so they have to make it lot more complicated and confusing than it needs to be.