What do i need to change on a 250 c15 bsa to convert it from 6v to a 12v and is it easy to change over?

There are a lot of English companies selling the technology and parts to do these conversions, which are very popular. I suggest you buy or get hold of a British Magazine called ; "Classic Bike" and check out all the advertisements in the mag. You are sure to find someone to help you,

I was able to do this conversion on a pre-unit Triumph 6T by tying together two wires coming off the alternator. It was so long ago (almost 30 years) that I don't recall if they ran to a rectifier, to the zener diode or what. I do recall that I read about the idea somewhere, but don't remember if it was in a Triumph restoration book, or somewhere in a ClassicBike magazine or something. Nor do I know if your alternator is similar enough to my Triumph alternator, but in any event, it should be a Lucas.

A good Brit mechanic would probably know, but there never were many of those in the States, and they would be pretty rare now. Try Britiron.com.

Of course, you will have to change your bulbs to 12v. I also put a ballast resistor in the feed to the coil to avoid having to dig up and fit a new one. I think that is about it.

p.s. If you know any Canadian telephone techs, you might try them as well. In my experience, they are miracle workers and geniuses when it comes to motorcycle electrics, but that's another story altogether.

To change a 6V to a 12V system you need to change every thing but the wires, every light , horn and electrical part.

There is no easy conversion for this. You would need to find a 12 volt alternator and voltage regulator made to fit inside this engine. As far as I know, no one makes these parts.