VO I'm not a diabetic, but my academic background gave me a decent amount of knowledge to understand the needs of diabetics. I think you just have to look at the caloric equation first:
Calories in = calories out --> maintain weight
Calories in < calories out --> loose weight
Calories in > calories out --> gain weight
I'm sure that after 15 years, and with the training you're already doing, you have an insulin regimine built into your daily life so you can exercise and eat without having any complications. Now the only thing to figure out is how you can create a caloric deficite.
There are two ways to do that:
1) Eat fewer calories while maintaining your current level of training - this should put you into caloric deficit and allow you to gradually loose weight.
2) Expend more calories through your training while maintaining caloric intake - same effect, except you're working on the other side of the equation and focusing on burning more calories than you're taking in.
I can't say from experience, though, exactly how to do this because I'm not sure of what you're eating and how you schedule insulin, training, etc. There's a lot that goes into it, as I'm sure you know, but basically if you choose one side of the caloric equation to focus on, then start gradually, and don't try to loose too much weight at one time. There are a bunch of considerations that one could go through when talking about exercising with diabetes, but I would first and foremost recommend going to a Registered Dietician or Nutritionist and talking with your doctor to see how to best make these changes while not putting yourself at risk for any diabetes related complications during this time. One thing I know about Type I diabetes is that it's important to maintain a schedule and keep things very similar from day to day (is that true?), so changing things to loose weight may or may not be a good idea.
Where are you from? I may be able to help you find resources for Registered Dieticians/Nutritionists in your area. Sorry I couldn't be more specific, but I don't have any first-hand experience with diabetes, except what I took away from my classes in my master's program, so all I can do is give suggestions and guidelines, and refer you to an RD and your doc. But hopefully some of this info was helpful and will give you an idea of where to go from here.