Is it more correct to say
a) John was appointed as manager of ACME.
b) John was appointed manager of ACME.
Or are they interchangeable?
Then there's also:
All three are interchangeable in terms of meaning, but the one without as or to be is the most common:
If you're looking for one to use, I would go for was appointed manager, just because it sounds more natural.
If you look for 'appoint' in online English-English dictionaries like Merriam-Webster, Longman, Collins, Macmillan or others you'll see both cases where 'as' is used and those where 'as' is omitted. To me absence of 'as' makes the appointment sound that much more solemn and serious, i.e. its more formal. But I guess its a matter of style and personal preference and both options can be used interchangeably.