Continuing to use it as a username is going to come across to a lot of people as, at best, deeply insensitive.
Actually this is the very first time I hear a comment about being nicknamed "Uncle Tom". So far in Europe it did not even started a single spark of debate.
If I appear to become a fashionable talent in North America I might consider changing the username to avoid controversy. At that stage I understand it can offend people. Now is a bit prematured in my opinion.
Thanks for raising the issue, I would have not figured that on my own :-)