Since C is your tonic chord, you probably wouldn't want to usurp it's position by converting it to a C dim 7th. According to conventional voice leading (the structure of the following chord), it would have nowhere to go that would make any kind of sense in the key of C—unless you wanted to modulate to another key.

Following the conventions for building diminished 7th chords and "spelling" them correctly, i.e., a pile of minor 3rds, or Root, minor 3rd, diminished 5th, and diminished 7th, here are various spellings of the same chord:

F# – A – C – Eb
A – C – Eb- Gb
C – Eb – Gb – Bbb
Eb – Gb – Bbb – Dbb

Insane, no?

In a sense, though, relative to the keys that they're in, they are not the same chord. It depends on the chord's function within the key. A diminished 7th chord needs to be resolved. One of the notes must function as a leading tone, that is, it must resolve to the root note of the major or minor chord that follows it (not necessarily the tonic). So that gives a clue as to how the chord needs to be spelled. What's the next chord?

I agree with Boromir. I see it as resolving to a G. The way the F# dim 7th relates to the key of C, resolving to any other chord (Bb, Db, or Fbb) strikes me as kind of goofy. A possible exception might be the Bb, which is the subdominant chord of the subdominant key (F), which means you've modulated. Slightly weird.

Don Firth