You are trying to do two things at once, which opens up the possibility of a confounding effect. You want to remove Wellbutrin to counteract the tendency to cause hypomania AND you want to add the antimanic effect of Lithium, or, do be more precise, the antimanic synergy between Lithium and Tegretol (see A lithium and carbamazepine combination in the treatment of bipolar disorder--a preliminary report. - PubMed - NCBI). Why not take it one step at a time and first remove Wellbutrin and see how you do on a monotherapy of Tegretol, and if you are still hypomanic, then consider adding Lithium into the mix?

You also need to look up drug interactions between Lithium and Tegretol - both are very old drugs with lots of side effects.