There's nothing wrong with converting to the thumb if thats what you want. I go back and forth between a quad and bike. Race the quad, pleasure ride the bike. Couple things about this:
1. Don't think changing to the thumb throttle is going to instantly make you a much better rider, it will help you, but it's not life changing.
2. Use a Honda
400EX or 450R throttle assy. About the best leverage ratio to reduce thumb fatique. Using the wrong throttle assy. on the FCR carb can result in a very stiff pull.
3. Some consider this the most important part...The twist throttle assy. on FCR carbs works with a two cable push/pull system. This is because the big fourstrokes move ALOT of air. There is the potential at WOT of having the slide "surf" the air flow coming throught the carb bell. I've ran the single cable set-up for a couple years and have never expeirenced a throttle hang, but the potential is there.
If you feel the change will help you, then go for it. To hell with what anyone else thinks!