If you have been playing for years like that it will almost certainly take a lot of effort to change it. If it were me I would just carry on with my usual practice routines (scales, chord changes, song practice but at slower tempos etc.) but while consciously holding the pick between the thumb and the index finger. You will almost certainly be tempted to swap back to your usual style of picking because it is more comfortable for you but aslong as you are mindful it will eventually become the new normal. There will not be a quick fix, just mindful repetition.

I would also try holding the pick the way you want to while watching tv or listening to music to increase your familiarity with it on more of an unconcious level.

Please feel free to take my advice with a grain of salt, as I have only been playing for a couple of months. I do have experience with successfully unlearning bad habits from other disciplines though, so that will hopefully lend my advice some credibility :).

I hope this helps a little.