Struggling to understand the new BBM update
...will I still be able to access my old chats or see old photos? 


No, they’re gone.

BBM messenger was shutdown May 31st, 2019 and users had until the following Friday to download messages, photos, videos, and files.

Source:  C|Net, BlackBerry Messenger shuts down: What you need to know

The “new” BBM app is their enterprise version and it’s completely different and is referred to in the article as a “replacement.”  Nothing was migrated from consumer to enterprise.