Depends what you are converting. Often people convert miniatures to look like some named characters that already have rules and points cost. If it isn't in a book, then generally people agree to rules with their opponents before a game. Don't make them too one-sidded though, otherwise people won't want to play with you!
For me, its usually an aesthetic thing rather than trying to make new rules and whatnot. For example, adding extra stuff or changing poses of current models to make them look more unique, or in the extreme making a whole model from something completely different but to represent that model. As long as you make it clear to your opponent what each model is.
For example. I bought some plastic skinks, cut their crests off and painted them earthy green as opposed to aquatic blue like me regular skinks. I use them as chameleon skinks because they are cheaper and I don't like the GW ones. I also converted a PP swamp troll into a slann, because it looks like a giant angry frog. It fits on a 50mm base and looks like what its supposed to be. So basically, if you convert something, you use it exactly the same as you would what it supposed to be. It just looks different, that's all.