High Flyer,
I did almost the same thing in June 04.
I have a CAA frozen ATPL with instructor rating and I wanted to get my FAA CFII.
Therefore I had to take the FAA Commercial written and IR written. Fly 3 hours with an FAA instructor so he could endorse me ready for the Commercial checkride with an Examiner.
I also had to fly another 3 hours with an instructor for the FAA IR endorsement, to show I'm ready for the IR test.
(The FAR AIM states 3 hours of flying required with an authorised instructor in the preceeding 60 days to the test.)
I took both flight tests at the same time and the IR side of the test was for me to demonstrate my proficiency since I already had an IR from an ICAO state.
After that, I then had a FAA Commercial/IR. But only on a single engine. If you want a multi commercial it'll have to be done in a twin.
I then had to do all the CFI, CFII and FOI writtens and get sign offs etc. And do the tests with an examiner.
So they will not just convert them.
So once you get as far as the Multi Commercial IR you will then go on for the type rating instead of the instructor route.
Luckily I didn't have to worry about the TSA last year.
If you a doing a type rating you'll need to wait until the TSA have approved you before you start the type rating.
It took me about 3 weeks to study, fly and take the tests for the Commercial IR.
Any more questions please PM me.
Hope this helps.