It depends on your region. In general these things take some time (hours) to go live globally to all enabled regions, and are done carefully. If you come back tomorrow or in a few days it's more likely to have reached you, assuming you're in an eligible region.

It's probably best to wait until the UI actually tells you Bard has been updated to Gemini Pro. Previous Bard updates have had UI announcements so I'd guess (but don't know for sure) that this would have similar.

> Bard with Gemini Pro is rolling out today in English for 170 countries/territories, with UK and European availability “in the near future.” Initially, Gemini Pro will power text-based prompts, with support for “other modalities coming soon.”

https://9to5google.com/2023/12/06/google-gemini-1-0/