To move the channel MUTE to be Pre Fader is a nontrivial task.
You need to open the Mute Switch section, S104B, from its circuit. Short the resulting open circuit and then insert S104B section into the S103A common leg.
1) Remove the Channel board
2) Cut the S104B traces for N.C. and Common at the 90 deg. bend adjacent to the N.C. terminal.
3) Scrape the solder mask from the cut,S104B- Common trace, directly across from the R129 solder pad.
4) Add a bare wire jumper from the Trace to R129.
5) Add a jumper from S104B-N.C. to S103-Common. Attach w/ RTV.
6) Add jumper from S104B-Common to JP102,103 "P" solder pad.You'll find this pad in the Fader area, next to S104B.
7) On the component side: Cut the trace between S103A-Common and R117.

Estimate is a full (uninterupted) man day to modify a 32/8. MEP

Mackie has a factory mod which moves the MUTE to the Post Insert, Pre-EQ position.

Correspondance # L128467 has a complete description of the above mod plus a mod to move the -20dB meter to PreMute.