You haven't mentioned the version of Word you're using and on which OS, but the following macro works for me in Word 2010 and 2013 on Windows:
Dim eqn As OMath Sub EquationSearchReplace() For Each eqn In ActiveDocument.OMaths eqn.ConvertToNormalText eqn.Range.Find.Execute FindText:="Det", ReplaceWith:="det", MatchCase:=True, Wrap:=wdFindContinue, Replace:=wdFindContinue eqn.ConvertToMathText Next End Sub
Description of all the parameters for the Find.Execute method can be found here.