Sort of an amplification of my earlier post, here’s an example of how I typically “reskin” a monster. This is an extremely simplistic reskin, and could actually use a bit more tweaking, but I wanted to use a “real world” example, not something I made up explicitly for this purpose. The original monster is on the right; the reskinned monster on the left. Even something as mildly changed as this cannot be done with the online “monster builder” tool. (Hell, I can’t even bring the damage in line with MM3 standards!)
As you can see — I added in a trait to make the “It’s a hobgoblin in a diving suit!” idea mechanically relevant, not just fluff; I added in the hobgoblin racial ability; I changed the damage and attacks to be in-line with the new standards; I changed the language and added a skill; and I edited the name throughout. Changes of this nature are the bare minimum of what I’d expect from a tool which allows you to “reskin” monsters.