All the types of substitution which apply to major keys (relative substitutions, scale tone triads or 7ths two degrees ahead in the key, dominant minor, and tritone substitutions) can also be applied to minor keys. The main difference you will find is that when you start to go into the upper extensions of chords built on the harmonic and melodic minors, you will get alterations to the chords. Here are the chords resulting from substituting scale tone 7th chords two ahead for the standard scale tone 7ths of A melodic minor.