Howdy! And Welcome!
As to changing from low carb to McDougall-eating and its effects on weight...I hope there are some with that experience here who can tell you what to expect...I've heard that just getting off the low-carb diet alone sometimes might cause sudden weight changes (usually gain????) in some individuals, but maybe somebody here has done so and knows for sure what to expect.
Over the longhaul, low-fat vegans usually have lower body weights than others, meaing Standard American Diet (SAD) followers, low-carbers, and even higher fat vegans and vegetarians.
There are varying degrees of McDougalling...it isn't too rigid...for examples, as Burgess noted, there is the Maximum Weight Loss (MWL) plan, in which everything is eaten in it's whole state, i.e., no ground up grains, even whole grains...etc. And then there is the regular McDougall program, which sticks to vegan and avoids oils...
...some McDougallers make desserts with whole grains, fruits, even sugar sometimes...others stick to more wholesome things and avoid all of that...it just depends on your needs, your health, how far you're willing to go, what you can stick with, if you're cooking for a family and children and such, and all of that. The more health concerns, the stricter a person would like to be in keeping to just basic whole foods.
I think his website and the newsletter archives give a pretty good basic idea of what McDougalling is, the variations for specific concerns, and how it applies to health problems and all of that. Also, you might find some helpful info in the Star McDougallers' bios also on that site. These are real life experiences related from indiviudals who have had remarkable results in weight loss and haulting or even sometimes reversing several types of chronic health problems.