If you are still running Server 2012 on any of your ConfigMgr site servers, now might be the time to start upgrading them or you’ll run into soe trouble when you try to upgrade ConfigMgr itself to 2403
ConfigMgr admins should start to see the warnings in their consoles informing them of the new requirementsÂ
Considering that Server 2012/2012 R2 went End Of Life last October, then Microsoft aren’t hanging around in getting admins to upgrade their servers to the latest OS (Server 2022 at the time of writing this)
Fortunately, especially if your site server is the one running Server 2012, you can do an in-place upgrade of the OS without having to rebuild your entire ConfigMgr estate – however, if you want to go to Server 2022, you will first need to go to 2016 or 2019 – then up to 2022
Anoop C Nair has an excellent guide on how to do this which can be found here