Some chord substitution "rules" are

1. For major:

   a. any chord of same series (type) with same root [e.g. CM13 for CM7]

    b. any chord whose root is a major 6th above the major chord and of
the minor series [e.g. Am7 for CM7]

   c. any chord whose root is a major 3rd above the root of the major
chord and of the minor series [e.g. Em7 for CM7]

   d. preceded by a chromatic neighbor above or below [e.g. BM7 or DbM7
to CM7]

   e. preceded by backcycling from its V7 chord [e.g. G7 to CM7]

2. For dominant:

   a. same as above [e.g. C13 for C7]

   b. same type whose root is a flat fifth above away [e.g. Gb7 to C7]

   c. same type whose roots are a b3rd apart [e.g. C7, Eb7, Gb7, A7]

   d. major type whose root is b7th of dominant [e.g. BbM7 for C7]

   e. minor type whose root is 5th of dominant [e.g. Gm7 for C7]

   f. minor 7(b5) whose root is a major 3rd of dominant [e.g. Em7(b5)
for C7]

   g. for 7(#5) [+7] or 7(b5): same types whose roots are whole steps
apart [e.g. C+7, D+7, E+7, Gb+7, Ab+7, Bb+7]

   h. and i. same as 1d. and 1e. [e.g. B7 or Db7 to C7, and G7 to C7]

3. For minor:

   a. any chord of same series

   b. any major chord whose root is b3rd above [e.g. EbM7 for Cm7]

   c. and d. same as 1d. and 1e. [e.g. Bm7 or C#m7 to Cm7 and G7 to Cm7]