The rating is a UK only "microlight" rating, and has equivalent status to say an IMC rating. This is irrespective of whether you want to fly 3-axis or flexwing.
You need to complete whatever training a microlight QFI considers necessary, then pass a microlight GFT. This is irrespective of whether you're going 3-axis or flexwing, but inevitably you'll need more hours for a flexwing conversion. You then send the paperwork off to the BMAA, who'll check it, forward to the CAA, who'll then return it with a microlight rating added.
To convert between 3-axis and flexwing, there's no legal requirement for a conversion, but it would be regarded as a survival trait. In this case, you need to do the necessary training, and pass an ACST (Alternative Control Systems Test).
I'm pretty certain that covers it, but the definitive source of wisdom (in office hours anyway) is the BMAA's licensing Guru, a chap called Roy Hart, on 01869-338888.
G