The rules and descriptions do not clearly define this here on Demiplane, or in the Druid PDF (AFAIF):
When does a Druid graduate from Tier 1 to Tier 2 to Tier 3 to Tier 4 for the Beastform Tiers?
Logically, I am assuming that it would follow the same Tiers of level progression (1, 2, 5, 8). Is this accurate?