I must say that almost all examples of replacing the first “as” with “so” concerned the adjective “long”. Can you shed some new light onto the matter? May any “as … as” phrase just as well turn into “so … as”? Or perhaps there are some rules behind it?
I think “as/so long as”, in the sense you mean, is a special idiom. In positive sentences you normally must use “as … as”. When it literally refers to length, “long” is no different (for example, “this rope is as long as that one”, never “this rope is so long as that one”). Another special case where “so” can be used is “as/so far as” (e.g. “He left, as/so far as I know”). It’s possible that there are other special cases too, but, as I say, the general rule is “as … as” in positive sentences.
“so … as” can be used in negative sentences (e.g. “He’s not so clever as he thinks”).
“so … as” can also be followed by "to " in sentences like “May I be so bold as to ask why?”, but this seems structurally different.
I think that SOME people have this explanation:
(a) Use as … as for positive sentences: I am AS tall AS Tony.
(b)Use so …as for negative sentences: I am not SO tall AS Tony.
(i) Most native speakers definitely do NOT follow this “rule.”
(ii) Some books explain that native speakers are so accustomed to hearing positive sentences with as … as that we can emphasize a
negative sentence better by using so … as.
(a) Although most native speakers do not follow this “rule,” I do. I think that it’s a really cool rule.
(i) I am not SO intelligent AS the other members of this forum, but I try my best.