First of all, a CAT 5 syllabus is code for "whatever makes sense to get you through the program." It's generally for O-5s/PCOs and other drug deals. If you were to go through as a "former" Blackhawk pilot, you'd most likely go through as a CAT 2C (Current), but with a modified syllabus since you didn't have previous experience that's applicable to the ATO phase. Spoiler alert, this modified syllabus, by definition, means it's a CAT 5 syllabus, but you'll most likely see it spelled out as "CAT 2C." Yes, it's a little confusing.
What does all that mean, in reality? Basically you'd go through the Pilot phase as a CAT 2C, but you'd go through the ATO (tactics) phase as a CAT 1. A typical CAT 2C is ~5 months, a CAT 1 is about 8 months, so the actual time will be somewhere in between. Having flown with Sierra transition guys and the odd Army Blackhawk person as an IP, I'd say you'd pick up the pilot phase stuff fairly quickly (basically your tailwheel and left main touch down differently on landing and autos will be "weird" due to weight/CG) but you would want to pay attention to the software stuff along the way, as it will appear to be designed by a retarded monkey. The ATO phase will be a lot to learn, but you'd be coming at it from a different perspective than a normal CAT 1.
If you're looking to apply to a HSM unit in the Reserves, there's really only two to go to and I've flown for and with both. One will be a training command unit (in San Diego) and getting in there will be tough as a SELRES and not really possible as FTS*. The other is an operational unit in Jax, which will be tough to pickup as a SELRES, but there's currently a former Guard pilot there now that did so, but her timing was somewhat unique and the Hawk pilot that applied after her didn't get picked up. FTS is a different matter. If going FTS, expect to go through the AC fleet training squadron on orders. If SELRES, expect the same, but there's a little more flexibility to do some of it in-house.
*Except when they need a body, which is now, but even then, not normally a first tour FTS billet for several reasons.