If you somehow take too much good bacteria then from what I've read the body just eliminates what it doesn't need. No issues.
It takes a lot of poor eating and poor food choice to cause the bad bacteria to dominate. By the same token it must equally take a long time to re-establish the good bacteria and create balance. So I don't think there is ever an issue of overdoing it with probiotics.
Dx mild pancolitis 2006