partial letter replacement in languages?

 (sorry...one posting of 'does language require deception to exist' is actually
this message, while the other posting really is about language and
deception...I mean no offense, and beg your pardons).



 is there a term for when a language is evolving/being changed, & replaces one
letter with another (ie, /d/ becomes /t/) in nearly all instances...yet there
are still words in the resultant language which retain (to continue the
example) /d/ ?

   thanks.

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