All we ever get from the movies is that Obi-wan used to be a Jedi Knight, but there’s so little information in the OT about the Jedi hierarchy that we really have no idea if this implies that he ‘ascended’ to the rank of master later on, hence the robe change, or that he is retired from the whole Jedi Knight business. Jedi Knight could have been an actual position in the Old Republic, which ceased to exist with the Republic. Or Jedi Knights are wholly different than Jedi Masters, like the difference between a soldier and a teacher.

I do believe that Knights and Masters had different robes to match their different roles.