Of course I have advice and some of it is actually meaningful. I wrote the following:
I have gone through this before. I would treat the "revised" system as a "new" system as you are going to have to go through all the tasks in your user guide to catch other changes, such as navigation and enhanced functionality. As you do that work, capture your new screenshots.
One other point. Does this system really have 900 unique screens? If so, WOW! If the 900 screenshots is because you have the same screenshot within multiple tasks, one strategy I would use would be to create a snippet to store that screenshot and then include a reference to that snippet. Most HATs have the functionality (RoboHelp, Flare, Confluence are the 3 I'm familiar with personally). If the same screenshot is currently in 10 different tasks, you would only need to update the snippet.
Sounds like a great project!
It sincerely sounds like awesomeness! I had this type of work with the rewrites of a trouble reporting system. There was Trouble I, then Trouble II, then Trouble II with RELTEC, then Trouble III, then, shortly before I left the company, there was Trouble Management. I never had 900 screenshots in my documentation for that system or any other system, except maybe the service orders system I documented. If I would have known then what I know now, the documentation for that service order system would never have looked the way it did. I was using RoboHelp, but didn't know to use snippets for screen captures - I had to work at Pearson to learn about snippets. Now, that said, for that specific service order system, there were not a lot of duplicate screenshots. Going by memory, I'd guess 95% of the screenshots in that documentation were unique screenshots and not used in more than one area.