Quitting during TC (and moving to another firm) is actually alot more common than many think. Don't be afraid to quit just because seniors or previous mentors told you that "it will burn bridges" or "you will have a black mark".

Literally nobody cares if you change TC, they just care where you spent most of your time. If you can get a better TC, or if your current TC really sucks, you should just change it.

Most bosses are understanding, and will want to keep good relationships with departing trainee/associates (especially given the size of the law industry).