Three simple steps for converting the Centipede swarm to a reasonable facsimile of the Viper swarm
Step 1: Switches- increase the Con from 8-12. Remember to adjust save DCs and Fort save in addition to hit points. Change venom damage from 1d4 Dex to 1d4 Con. Give it a good Reflex save, because it's now an animal.
Step 2: Removals- take out darkvision, tremorsense and vermin traits.
Step 3: Additions- add Intelligence 1, and skills and feats to go with it (hint: swarms don't need Weapon Finesse, which the centipede swarm for some reason has)
You might consider raising the CR of the swarm by 1, because of the additional hit points. It's up to you.
Note: This is not the Fiend Folio's viper swarm. It has more Hit Dice, for one thing. I actually prefer this little palette swap.
Demiurge out.