BTDT - God help them
About 15 years ago I worked on much the same project, get a HR/Payroll system for a large Gubmint organization off a 24/7/365 mainframe system to a shiny new ERP based system. I was brought in because the original consultants for the project were dumping more and more verticals from HR into the mix despite having little or no experience with the software. This isn't the end of the world but you should note that while a couple of buckshot pellets may not bring down a duck, it will wound them seriously enough to not withstand the next volley of pellets. Did I mention it took almost 3 years? It did.
Their real problem was with the architecture and the "experienced" consulting company they brought in. First off, they insisted on a 32-bit architecture in Windows and an older version of the underlying database. Next off, considerable structural modifications were made to "proprietary" internal structures of the vendors software. The consulting company itself had little taste for doing anything other than managing the project and hiring their own contractors to perform actual tasks like coding, training or troubleshooting. (That's why a Big 4 (US) isn't always a great choice)
At this point I was brought in to keep them honest and provide for a differing viewpoint. As soon as they figured that out I was excluded from meetings. This is when I first suspected that the client was powerless for reasons that extended past the contract negotiations. As the client kept rearranging the deck chairs, their ability to dictate events became even less. Another point, Gubmint IT is hopelessly outclassed when negotiating contracts with large companies. I don't know how many surcharges for work not specifically in the contract were billed but they were considerable.
One of their most egregious system mistakes was a global search system in 32 bit windows. This despite every expert other than the consultants telling them it will be impossible to maintain and keep running. What happens is the indexes used for search have only a 2GB filesize. You have to allocate files before it grows to the limit otherwise you have to rebuild the search DB from scratch which takes a few days. Of course this is exactly what happened over and over again. I believe this was a big reason for the later upgrades.
So after 68 million for the system they were left with a crippled system that was only online for users for 12 hours a day during the week. A far cry from the 168 hours a week with the mainframe.
At this point I was deemed too expensive and in truth had already had a job offer to start in a month. They later tried to bring me back but I was engaged and the pay offered was about 30% less than what I'd want to go back. Finally I'd quoted them at 150% of my last rate as a starting per hour rate and hoped that would have them go away. They did.
Did I mention the project took almost 3 years and the version they were on was no longer supported at the end? They brought in the software vendor to perform the upgrade and it cost another 15 million to upgrade and fix all the modifications done to their proprietary structures. I think they allow users on during the weekends now. So that additional 24 hours was probably worth the 15 million.