Ask HN: How to manage legacy knowledge and experts

I work at an enterprise software house, where we have two old (20+ year) applications that are making us shit-tons of money despite the spaghetti code that has developed over the years.

We have around 5 developers who have been with use for 10+ years who know everything there is to know about it. It's very buggy of course, and we have a substantial 3-layer support team.

The problem is that, although there is lots of documentation, it gets out-of-date quickly and doesn't actually define the system or how to fix problems. So the 5 experts are needed so that he newbies can have a chance of addressing issues within the SLAs. Without these 5, we are screwed.

If they chose, they could hold us to ransom. they are excellent, professional individuals who wouldn't do this .. but there is a risk.

We COULD have those guys spend 6 months documenting the code, and what it's supposed to do. We COULD get them to set out structured training for new recruits .. .but these 5 are developers not trainers.

Are there any companies that provide expert consultancy for this kind of thing? A company who would come in and tell us how to do this effectively? If there aren't any then I'm totally going to set one up.