It is said that, before the Second World War, there were curious differences in the writings on signboards of pawnshops, which seemed to differ based on the prefecture one was in. If you were in Tokyo, you might see 「しちや」 (shichiya). But if you were in the Kansai region, particularly Osaka, you might see 「ひちや」 (hichiya). You would also hear differences in how certain words are said, such as 七 (seven) being pronounced as shichi in areas like Tokyo, but hichi in places like Osaka. So what is going on?
This pattern of /s/ -> /h/, and to a lesser extent, /h/ -> /s/ changes are documented throughout Japan, showing up in various dialects and variants spoken throughout. But why does this occur? One leading theory is that the /h/ sound is way easier to pronounce than /s/, and by the preference for least effort in pronunciation, this has entered colloquial usage. However, this does not explain why some /h/ sounds are interchangeably pronounced as /s/.
So what examples are there?
Firstly, let us go through the /s/ -> /h/ mix up, which comprise most of the cases of interchangeability here. There are also place names with this /s/ to /h/ interchangeability as part of the official name. The hotel 七福旅館 in Tottori Prefecture is read as ひちふくりょかん (hichifuku-ryokan) and a town called 七宗町 in Gifu Prefecture is read as ひちそうちょう (hichisouchou).
Kansai-ben, the dialect of Japanese spoken in prefectures like Mie, Nara, Kyoto, and Osaka, is known for such interchangeability as well. Examples of this include some phrases such as はん (han) from さん (san, the marker attached to last names when referring to people), へん (hen) from せん (sen) in ません (masen, the negative form of a verb in formal speech), and はる (haru) from さる (saru) in なさる (nasaru, another word for the verb to do). More extreme examples also include a degree of contraction, such as this expression hona (if that is the case) in Kansai-ben:
それなら → そんなら → ほんなら → ほな
sorenara → sonnara → honnara → hona
This interchangeability is also seen in other Japanese dialects like Tsugaru-ben spoken in the western parts of Aomori Prefecture. Here the /s/ sound regularly becomes /h/, and so for words like sensei (teacher), you might hear henhe instead.
Now that we have covered some examples of the /s/ to /h/ interchangeability, what about examples of the converse? Well, while these are more common in East Japan, examples of these are rare, and you would not quite hear the /h/ to /s/ interchangeability in cities like Tokyo. One example here is 冷やっこい (hiyakkoi), another word for 冷たい (tsumetai), which means “cold”. In Tohoku-ben, a dialect of Japanese spoken in prefectures like Iwate, Aomori, and Akita, in the northeastern parts of Honshu, this can be pronounced as しゃっこい (shakkoi) or しゃっけ (shakke).
So this has been a little exploration into the various ways two consonants could be interchangeably used, although the cause for this phenomenon has not really been properly established. These variations generally show no proper spatial pattern between where certain dialects are predominantly spoken, so whether or not this may be historical, or just regional, is up for linguists to research.
For learners of Japanese, does this phenomenon really matter much to your learning? I would say, not really. You can still get by with the Standard Japanese taught in lessons, books, and things like that, but I think it is nice to take note when travelling out to the other prefectures in Japan.