I'm changing a song (When I Was Your Man) from a C major to C harmonic minor. I am not quite sure what to do with the Dm Em and Bb that exist in the major key. I have been using Ddim which has worked fine enough, but I have no ideas on how to handle the Em and Bb. What chords might work best in their place?
I'd generally substitute E♭ chords for the Em chords, since both correspond to mediants of their respective keys (C minor for E♭ and C major for Em).
I'd leave the B♭ chords well alone since they're common in C minor; I do not recommend using harmonic minor alone when changing tunes from major to minor.
If you insist you must use harmonic minor, then E♭ augmented and B° are your only options.
Don't worry too much about whether it's in the melodic, harmonic or natural minor scales. You don't have to stick to diatonic chords. The original version didn't - it was in C but included Bb!
Modify the melody to have flattened 3rds and maybe flattened 6ths. It seems there are some flattened 7ths there already. You may not have to change as many chords as you imagine. You're giving the song a different flavour, that's all.
Those basic chords in minor would be
iio III VI or
d diminished, Eb major, and Bb major.
As @LaurencePayne pointed out the
Bb major is already used in the original form of the song.
I imagine you are getting stuck when trying to use
III+ (Eb augmented.) If so, that's understandable, because the mediant chord in minor is normally a major chord not an augmented chord. (Minor key harmony isn't based simply on one form of minor scale, but that's another story.)
iio is working OK, great. You could also try the minor