I've put what I know in your text below:
First there was the Bell 222 A.
What did they change to make the 222 B?
The difference between the A and B is the engine and electrical. The B model has the LTS-101-750 vice the A's -650 engine. There was also some changes to the electrical bus.
Then what did they change to make the 222 UT?
The 222UT removes the wheeled landing gear and replaces it with skids. There is also changes made to the electrical system and I also believe this model can fly SPIFR without a SAS system. It also comes standard with the utility interior.
Then the 222 SP?
The 222SP is an A model that has the LST-101 engine removed and replaced with an RR/Allison 250-C30 engine installed. This was done by a third party outside of Bell under an STC
Then the 230?
The 230 is Bell's answer to replacing the LTS-101 with the RR/Allison C30
Then the 430?
Bell added the 4 bladed rotor and more electrical improvements and I think the cabin has a slight stretch.
Is this the right progression? Can any 222, 230, 430 be retro fitted with skids
as an STC or did it have to come from the factory like that?
There may be a way to change from wheeled to skids but most come from the facory that way. I think the cost to do so would be too high though.
Can the 222 A be made into a B or UT?
No, why would you?
Can the 222 B or UT be made into an SP?
Yes, reference STC SH7853SW
Does the SP and 230 really outperform the 222B and UT?
Can a 222 be made into a 230?
No, but you can put the 230's engine in the 222