I too had been diagnosed major depressive disorder for years, then last year I "became" bipolar 2. We started noticing that some of the "good" times were more like hypomania, lot of the racing thoughts, energy, doing everything. (2 months ago I impulsively adopted a kitten, and I love her). Now instead of just being on antidepressants they add a mood stabilizing agent (right now it's the atypical antipsychotic Geodon). I'm not doing too well right now, in the depression area, so I'm not sure what it will be next.
Anyway, yes, you can be "switched" from unipolar depression to bipolar. Take care. HUGS!