S. medial [x] became [h] in Gondorian pronunciation; [-x-|xt] > [-h-|xt]
In the Gondorian dialect of Sindarin, any ch [x] was reduced to h [h] medially. This is a change Tolkien mentioned numerous times, invariably in the context of the name S. Rohan and related words:
CH is only used to represent the sound heard in bach (in German or Welsh), not that in English church. Except at the end of words and before t this sound was weakened to h in the speech of Gondor, and that change has been recognized in a few names, such as Rohan, Rohirrim (LotR/1113).
In the pronunciation of Gondor the ch (as in German, Welsh, etc.) had been softened to a sounded h; so in Rochann “Hippia” to Rohan (Let/178).
Its strictly correct form was Rochann, but the form Rohan represents the actual pronunciation of Gondor, in which medial ch was colloquially weakened to h (PM/53).
The language of the Rohirrim contained the sound here represented by ch (a back spirant as ch in Welsh), and, though it was infrequent in the middle of words between vowels, it presented them with no difficulty. But the Common Speech did not possess it, and in pronouncing Sindarin (in which it was very frequent) the People of Gondor, unless learned, represented it by h in the middle of words and by k at the end of them (where it was most forcibly pronounced in correct Sindarin). Thus arose the names Rohan and Rohirrim as used in The Lord of the Rings (UT/319).
As noted above, this adaptation was because the common Westron speech did not contain the Sindarin sound ch [x], though it is also quite likely Tolkien’s true motive was to simplify pronunciation for English speakers. The last quote indicates the pronunciation became [k] finally, but that is the only places this second sound change was mentioned.