Is it possible to be diagnosed as bipolar, then3 years later have different symptoms new diagnosis is major depressive disorder no mania present?

Bipolar: Yes it's possible, especially if incomplete history is taken. Bipolar disorder is a comprehensive diagnosis that considers your whole history. It's not "just" depression, but is a mood stability problem. You could be hypomanic/manic at one time, & depressed at another. If there have been previous episodes of mania/hypomania & you are now depressed, it's still bipolar disorder though.

Yes: The first diagnosis could be wrong - sometimes people can have anxiety or emotional instability that looks like bipolar disorder but is not. Most times, people start with a diagnosis of depression, maybe for years then it gets changed when the person has their first manic episode. A manic episode defines bipolar disorder.

Unlikely: Either the original diagnosis was wrong or the new one. By definition bipolar includes hypomania or mania. Decreased need for sleep, impulsive behavior, expansive mood, or even irritability are manic type symptoms. There are also mixed episodes in bipolar. Mdd is only depression.