What would you do if the codebase was actually 5 codebases absorbed from 5 different smaller companies? Assume that zero institutional knowledge about the code / business have been passed on.
I have to assume someone is using the software therefore there is some tribal knowledge of what it does? Otherwise this is maybe SAAS software that users use and some functionality is exposed that would allow you to begin decomposing backwards toward expected input/output. You're almost black-boxing at that point.
I will admit that I have, on very rare occasion, scream tested a piece of software running on a server that nobody would claim ownership or knowledge of either on the eng. team or within the org.
Who is, in that case, using the software? They obviously understand the context by which the software is at least going to work, otherwise, why is the software being rewritten?
Who is requesting the rewrite? Do they know what it is supposed to do? Is there an executable build of it that exists somewhere?
Rewrite is being pushed by certain parties because we're unable to meet feature requests quickly with the existing system, and it's being assumed that a rewrite will fix that problem. The team is barely functional though (from the top down). I've seen a few failed projects now and I don't think the rewrite will ever be accomplished. If we manage to rewrite, it's far from certain that we'll do a better job than the last guys did.