C.E. Evans wrote:
It seems way easier to just place McCoy's arrival in 2266 after Doctor Piper's tenure as CMO than to retcon "The Menagerie."
If Crusher were asked how long she had served as CMO on the E-D, would she care to count the year (season 2) she was "on sabbatical"? Perhaps McCoy took a leave of absence and Piper was his replacement during that time.
Only if you really personally want to place Star Trek VI in 2291, otherwise it's easily a straightforward case that McCoy succeeded Piper as Enterprise
CMO in 2266 and served in that capacity for 27 years before moving on in 2293.