I once worked with a client in such a situation. They were in the process of building a huge oil rig, costing somewhere north of $5.5bn USD. The basic premise was that a previous vendor had configured a big documentation system running on a soon-to-be-outdated Windows Server version ages ago, along with a "kind of" API allowing the shipyard to send information in the form of equipment/construction metadata, documents of various kinds and similar stuff.

The main challenge was that nothing could not be resent if a transmission failed, unless there was an actual change on the shipyard side, or by an highly complex and manual method. There was no source code to anything, and while the enterpricy system was a fairly standard off-the-shelf type thing, the API was completely customized. Nobody knew anything about how it talked to the system, dependencies or anything else. Changing anything was out of the question, as everything had been defined in contracts and processes making waterfall seem agile.

The team I was in was basically there to set up a new application in a separate environment, so that we could migrate and replace the existing setup once the shipyard had handed over all the information. In the end everything was kind of anti-climactic, with everything working as expected for as long as it needed to.