Then... you can no longer spend pearls unless you undo some of the improvements. 😁

What exactly are you asking here? I don't think you "unlock" something or do anything similar. Pretty sure you're just done, same as with buildings/discoveries upgrades.

Once I know what it is you're asking, I can check with someone.

any bonus  ???

I just checked and got my answer: nothing happens. No bonuses nor anything else. You're just done.

Thank You.

Hello Captain @Master Blaster , just a suggest, sometimes there are events about using your pearls at Witch Doctor's Hut, you can [ Edit ] and re-do apply pearls to get some rewards on these events, i always do... well, it isn't much but it is better than nothing.