When to stop using a Fungi for tanking.
 

I have a DE SK and was wondering when I should look into getting another bp like the Incarnadine Bp and use that instead of the fungi when I'm tanking for a group?

When you get your Will of Innoruuk?

I would say that Dark Elf SK has no need of changing out the Fungi, ever. You can easily reach your AC cap through other plate armors; it's not like one of the other slots you'd be replacing will have the supreme advantage that +15 regeneration affords. And it's decent AC to begin with.