Another method would be a simple IF field with AutoText fields.
I am assuming a legacy drop-down field with a bookmark and set to calculate on exit.

{ IF "{ REF DropDownBookmarkName }" = "yes" { AutoText "AutoTextEntry1" } { AutoText "AutoTextEntry2" } }

Note, the field braces { } cannot be typed in from the keyboard. You have to use either Ctrl-F9 or the Insert > Quick Parts > Field.