Back to the original post. How much would cost and how long will it take to get the conversion to FAA?
Two written tests, IR and commercial.
IR check-ride in a single engine. For IR, it doesn't matter if you do it in a single or a multi in FAA land.
And then, it depends on what you are after.
If you need commercial privileges for both single engine and multi engine, you need to do 2 check-rides, one in a single engine, and another in a multi-engine.
Some DPEs might be willing to give you a check-ride for both commercial and IR at the same time.
I suggest you fly with an instructor for a few hours in each category of airplane and prepare for the oral.
