[enyafriend]

It don't just happen like that. It is not as simple as how you put it i.e. a "polite" to "casual" speech transition. One could be working together as co-worker for a year or two (or even more) and still be on the polite side. Same for the bowling mates that you meet every weekend.

The rule is to adhere to the senior/junior relationship closely. This is where you separate yourself between speaking polite or casual. And this is just part of the rule. There are plenty more.