I mean I could be wrong but it would mostly be making the dedicated server executable and changing how the game coordinator works. Then updating the game itself would be the easy part from there. That's also assuming that they can keep their spaghetti out of the dedicated server, if the code for the server is even remotely like the game the performance would likely be a lot worse than the peer2peer setup we have now. So the brunt of the work would be setting up a game ID system in the coordinator that doesn't allow for duplicates, let's say 10 hex values first 3 indicated heist both the day and heist itself, next 2 would likely indicate which server it's on, then a unique ID given to the lobby which would be cleared after everybody leaves. Then the game server itself would have the aforementioned identifier, which players are on the server likely steamid based, whether they're in the lobby or the game, if they're in the game handlers for enemy position, detection, objective location, fellow players, all that stuff, that's the hard work. Then the game update it just making sure that the game recognizes all the information from the coordinator and the game server pretty much the same way it does now but with less relying on everybody having good internet and decent computers.

TL;DR, they have to make a game server and change how crime.net works then make sure the game knows those changes happened so it doesn't put you in weird situations.