Word Usage – ‘Back in the Day’ vs ‘Back in the Days’: Understanding the Difference

For the most part, the two phrases can be used interchangeably and will get the same point across. However, there is some nuance between the meanings of each expression. "As a result of X" implies that there's are other results of X. "As the result" implies there is only one result. I believe this is the point you were making with respect to "definiteness." I initially disagreed with the statement that "as the result of" is seldom used, but an Ngram supports that the expression with "the" is used 10x less than the one with "a."

The difference is in the degree of change applied to the object.

Enhancing leaves the main structure and functionality the same, most features intact, and generally speaking improves the object by applying small modifications.

Revamping often changes the basis, some fundamentals, some key features. Structure can undergo some changes, but the functionality generally is kept as is although we can see re-prioritizing of some elements.

Overhauling involves disassembling into constituent parts, examining each of them carefully for flaws, replacing with equivalent new parts (subsystems), in order to bring the system's performance closer to the original specification/idea.

Those words are not interchangeable. You could overhaul an engine of an aircraft to bring the engine back to its "factory new" performance. You could enhance an engine of an aircraft by replacing some heavier parts with lighter ones, for instance. You wouldn't revamp an engine. The latter most often used to describe a process for making fixes in a social/business system, like government, company management, etc. You could, however, revamp a design of an engine, which often includes technological part (manufacturing).