Bipolar I and Bipolar II are separated mainly by the severity of the elevated-mood episode, not by how much depression is involved.
The defining difference
Bipolar I is defined by at least one full manic episode, severe enough to disrupt daily life, sometimes requiring hospitalisation, and sometimes including psychotic features. Bipolar II is defined by at least one hypomanic episode (milder, shorter, without psychosis) plus at least one major depressive episode.
Depression is not the dividing line
A common misconception is that Bipolar II is "just the depression side" of bipolar disorder. In practice, the depressive episodes in Bipolar II are often just as severe, and sometimes more frequent, than in Bipolar I. What makes Bipolar II distinct is the absence of full mania, not a lighter overall course.
Can a diagnosis change?
Yes. If someone previously diagnosed with Bipolar II later experiences a full manic episode, meeting the more severe criteria, the diagnosis is typically updated to Bipolar I. The reverse doesn't really happen, since a full manic episode in someone's history already qualifies for Bipolar I going forward.
Why the distinction matters for treatment
Treatment approaches, and how closely mood needs to be monitored, can differ between the two, which is part of why an accurate history (not just current symptoms) matters so much during diagnosis. This is one of several reasons a full clinical interview, rather than a short self-check, is needed to reach an actual diagnosis.