If you were to attempt to tackle a project of this magnitude you wouldn't be able to get away with it being left up to individuals. For balance purposes it would have to be organized into teams, otherwise you run the risk of making thing wildly
OP.
At the very least, you would need an impartial editor or two to give feedback/final approval.
First off, each entry for the current codex would need a brief summary of five main points:
• price change
• stat change
• equipment change
• rule change
• other
(obviously, not ever unit would need every point adjusted, so simply leaving it blank or "N/A" would be fine)
So using Destroyers as an example,
NECRON DESTROYERS
Price: decrease (35pts)
Stats: N/A
Equip: add weapon options
Rules: add option to take equip that gives Relentless
Other: N/A
For Flayed Ones,
FLAYED ONES
Price: N/A
Stats: N/A
Equip: N/A
Rules: give Shred (another rule so possible to assault after Deep Striking?)
Other: move to Troops. change Imotekh's Bloodswarm Nano-Scarabs so that marked unit is chosen by player, no longer random
Once the entire codex has been evaluated and everyone has weighed in on what's fair/what needs to be changed, THEN you start working on the fine details.