How to convert and use 10mm dia steel instead of 12mm dia steel which is spaced at 200mm center to center?

No need to convert. the centre of both sizes will always be in the same position, irrespective of how wide the diameter is... up to 200mm diameter steel.

I'm guessing that you are changing the re-bar specification for cast concrete... might I suggest that you clamp together 2 long bits of 2 x 2 or 3 x 2 along their lengths, and then drill a 10mm hole at 200mm centres along the join... then unclamp and lay apart... repeat for bars cutting across at 90 degrees... then simply drop the re-bar into the now 'half' holes.. far quicker than measure and tie, measure and tie, etc.