As with always, moving from a snaffle to a curb takes time and adjustment. Personally, I think there are better curbs out there. (For most horses)
I don't really know how to answer the question "is it easy" though. I think it would be easier to switch to a different curb bit as many horses don't quite like tom thumbs. (Western ones, apparently english tom thumbs are mild? Don't quote me there)
If I want to switch a bit, I just switch it. I will lunge them in a bridle for a few minutes, and then just ride. I try to be lighter with my hands and I don't expect her to behave the way she usually does. While it may take time to get used to a new bit and respond "correctly" to it, you don't have to gradually do it. That would be like gradually introducing an english saddle to horse that's only had a western saddle on it. No big deal.
As far as the bit itself, I'm no help. I ride Ricci in a rubber bit.
I agree...why a Tom Thumb? My aunt had a mare that was really soft-mouthed and she used a Tom Thumb on her...she threw her head every time you put pressure on the bit. After that, I put my own snaffle on her and she was a dream.
I ride Sam in either a french link loose ring snaffle or my birthday present - a Comfort Snaffle w/ copper roller HBT shanked Myler. He reacts no differently with the Myler than with the Loose Ring. I do like the Myler bits, I have to agree that that would be worth looking into - although FYI, they can be quite spendy.