I would like to know more about the distinction between the use of the instrumental (творительный) and nominative (именительный) case in the context of a predicate (сказуемое) parts of sentences in combination with the (linking?) verb быть.
As far as I understand, there exists a subtle semantic difference between, e.g., мы были друзья versus мы были друзьями. I understood that this difference is mainly temporal: the former case expressing staticity, while the latter
Although I could not easily find any formal rules in academic grammar books (for example in Shvedova, 1980); I found a couple of academic papers that state that there is some 'friction' between the two forms. This makes me wonder whether these forms are sometimes used or interpreted interchangeably (albeit incorrect), and whether this is some sort of ongoing language change or not.
I understand that a few related questions exist already on this stackexchange, however, I am specifically interested in the 'language change'-aspect of it, which was not asked on this stackexchange before.
I would really love to hear answers from the community!