How do I replace bolts for my toilet that were concreted into the original flange?
If you have access from below it really is a quick job to put a new flange on.  But honestly if the one you have is that solid and has worked well (doesn't clog a lot), then I would simply just throw on a bolt extender.



Note:  Your bolt looks like it extends pretty far so not sure what toilet needs more than that but gave you an answer.