See, that's even better. You don't want to screw with a desk with six aux sends, four EQ with two sweepable mids, group assigns etc. I mean, yeah, you wanna mix on a desk like that, but follow along:
Upgrade like two channels, leave the others stock. Pimp out the power supply. Review the grounding scheme and improve as necessary. Upgrade the master section.
Now, if this board has individual channel strip PCBs (somehow I gathered that it did), strip out two of them, and start thinking about something you'd like to stuff in their place instead. There are lots of preamp schemos out there, you could stuff in some serious cool EQ, etc.
One thing I hate about most cheap live desks is the EQ at 80Hz, 2kHz, and 12kHz, and a low cut at 75Hz. That gives me exactly two useful controls: 2kHz and the low cut switch. I say tailor those points to what you want to mix: I'd use 200Hz, 3kHz, and 8kHz shelf, along with the low cut. A sweepable mid is cool, but if I had an extra pot hole, I'd use it to stick in another band instead, probably 1kHz. I would tailor the Q differently for each band, but that's just me. Man, if I had that EQ for the gigs I normally do (church, school plays, etc.), I would weep tears of joy.
It will take you a long while to figure out and design what you need, but in the meantime you'll have two upgraded channels, four old channels, and an upgraded master. In the future, when you've fully pimped out all eight channels with your custom strip, you will rule the world! And when you're done, you'll understand waaaay more about electronics than opamp and cap swaps will teach you. And it will beat the crap out of spending $50-$100 on opamps and then deciding when you're done that the result is still just OK.
