When can milk substitute for buttermilk in sourdough recipes? I want to prepare buttermilk sourdough pancakes:

One part sourdough starter
Two parts flour
Two parts buttermilk
Sugar to taste (about a quarter part, depending on mood)

The idea is to leave it overnight and let the sourdough SCOBY incorporate the buttermilk culture. In the morning, the sponge can be used for pancakes, waffles, etc. At a chemical level, the yeast eat the flour and make sugars and carbon dioxide bubbles, the lactic acid bacteria eat lactose and other sugars and make lactic acid, the overall pH drops from a milky 8 to a sour 4, and the buttermilk effectively spoils but in a very controlled fashion.
I'm currently snowed in due to a blizzard. I don't have any buttermilk, but I do have some milk. From a chemist's perspective, surely the desired reaction will proceed eventually, and the only changes will be how long it takes for the culture to rise and sweeten, and the overall proportions of lactic acid bacteria. However, from a cooking perspective, those two things matter quite a bit. What are the consequences of substituting milk for buttermilk in sourdough recipes?
As a practical matter, I do have some lemon juice and can sour milk into buttermilk using that common combination. I'm mostly curious about whether there's a flavor or safety reason for preferring prepared buttermilk.
I think your understanding here is incorrect.

The idea is to leave it overnight and let the sourdough SCOBY incorporate the buttermilk culture.

With a mature, functioning starter, there won't be much of any &quot;incorporation&quot; overnight, in the sense of actually reaching a new equilibrium of species in the starter. I don't think the buttermilk is being added to change your starter, it is just there because it is a common ingredient for pancakes and the like.
So, just go ahead and do whatever substitution you like that would be appropriate for the same batter without sourdough. You can expect analogous results, maybe even closer to the original, since you already have some lactic acid production from the sourdough.

curious about whether there's a flavor or safety reason for preferring prepared buttermilk

Flavor yes, buttermilk tastes differently from buttermilk substitutes. Safety no, adding buttermilk to a batter doesn't make it shelf stable.