I only qualified in shellac a month ago so don't want to be jumping ahead into loads of training just yet.
I don't think you need to do a conversion course, Gelish is fairly simple. I've been using it for years, here are my steps (all after basic pep as I don't scuff the nail plate):
1. Scrub nail with scrubfresh until they squeak.
2. Apply Foundation, wiggling the brush into the nail as you go. This scrubs the base coat into the nail so it adheres better. Cure UV 60secs
3. Dry brush. I used to not to this and had a load of shrinking problems. Switched back to dry brushing and they all went away.
4. Apply color coats. Curing in between at 2min each coat
5. Apply Top it off thinly. Cure 2min
6. Remove tacky layer and apply oil.
I think the major problem with those who use Shellac and go to another gel polish brand is they apply too thinly. Shellacs "thin to win" doesn't work well with other brands. I apply just like I would regular nail polish and thats perfect. I used to use Shellac and would apply Gelish as thin and it would never last.
Another trick is on the second color coat, you dont have to cap the free edge. This reduces the bulk and I find it lasts longer.