Aircraft are type-certificated under various categories. For example, a Cessna 172 is in the Normal and Utility categories, and operating limitations depend on what you’re doing with it. A Beechcraft Bonanza is in the Normal category. Homebuilts are under the Experimental category, and some ex-military aircraft (and others) are in the Restricted category.
I see a lot of Bell UH-1s that are in the Restricted category. But UH-1s are Bell 204s and Bell 205s, which are in the Transport category. Can a UH-1, or any military aircraft that has a civilian counterpart, be re-certified into a different category? If not, why not?
AFAIK, once an aircraft is in a category it stays there; but I don’t know for sure.
Takes a lot of testing.
I used to be around that activity back in the day & it was not easy then. Today, it is probably harder & more expensive.
It can be done in some cases.
Usually cheaper to buy an aircraft that is designed for the activity you want.
Air racing, fire fighting, moving large aircraft from passenger work to freight work will usually be worth the effort, especially if you can do it out of the USA where the rules are easier to work with.
To general a question IMO, you need to narrow it down a bit. What do you want to do?
Not something I want to do; just curious about it. If it was something I wanted to (actually) do, I’d have the money to do it and wouldn’t worry about it! 
But for the sake of this thread, let’s say someone gave me a UH-1B, which is a Bell 204. It’s got the Restricted placard on it. Since the 204 has a type certificate for Transport category, and a UH-1B is a 204, then could the UH-1B be made to conform exactly to the 204 type certificate and be re-certified as Transport? Or would it be possible to move it from the Restricted category to the Experimental category?
FWIW, I know that many (most?) T-34s are in the Normal category, under the Beech 45 type certificate, and that the Sikorsky S300 is the same as the Schweizer 300, which is the same as the Hughes 300, which was certified as the Hughes 269. I know the UH-60’s civilian name is the S-70.
There was an article about exactly this in Air & Space magazine a few months ago. To summarize from memory, because I don’t keep them when I finish them, Yes. But in many cases, it’s too expensive to be worthwhile, and in a few cases, it’s utterly impossible, because the different versions are too different. You’d essentially be remanufacturing the entire aircraft.
ETA: The article wasn’t primarily about this, it was about something else, but for the purpose of the article, it was necessary to have a few paragraphs explaining why it was impossible for the aircraft the article was about.
Say the military specs a certain rivet type in a structural member & ther civillian aircraft was certifies with a different type of rivet the FAA likes but the military does not, you would have to replace all those or get the FAA to change their mind. Bawhahaha
Remember, it is up to you to prove what you want to do is good & contrary to most of the PR, the actual FAA field men are not so willing to just make it easy for you…
If it is just for you, then experimental will do just fine.
But
If you want all xyz helios to be utility or normal, then it is not about you , but passengers, Part 135 and all the other t5hings others might want to do.
They are not going to be nice and let you do that just to your airplane, that precedent would be a real disaster for them.We wanted to use a different material for baffles & the local guy looked it all over and said OK but is was done on a single aircraft basis, I forget what we called it nopw & I’m too lazy to look it up. But he told us that if we tried to do it to others with a 337 using our plane as the testbed, then we were looking at a BIG deal…
Prolly get real good answers to this and all FAA questions if you call your local FAA flight safety officer. Or maintenance officer. If you are not on a first name basis with one of them & you want to work on the funky fringes of the aviation world, that is one of the first things you do.
My info is at least 20 years old but I do not think it has gotten any easier…
No doubt. But I’d hate to bother them with just idle curiosity.
‘At the FAA, we’re not happy until you’re unhappy.’
I think you gave me credit for a quote I did not do. LOL
Second sentence is what I have experienced some. 