Upgrading to another version of Windows is fully supported by Microsoft but need to check the OS or operating system upgrade path.
The architecture platform has to be the same, which means (x86) 32 bit to 32 bit of OS and 64 bit to 64 bit OS.
Upgrade of OS to 32 bit to 64 bit cannot be done.
Upgrading an OS to another version will take a short time if no hiccups during the upgrade.
I had tried upgrading a Win 2008 Standard to Win 2008 Enterprise; I was thinking that it will take only an hour to do the upgrade but it takes more than an hour.
The SP2 installed on Win 2008 Standard has preventing the upgrade.
The in-place upgrade shows that the "Upgrade has been disabled" and its grayed out and not clickable.
After removing SP2, the option to upgrade was enabled and it goes smoothly.
If not able to remove SP2, arrange the updates by date and remove some updates prior to SP2 and after SP2.
But before doing the upgrade, if you have a spare machine to clone the server and do an upgrade and test on the clone machine would be the best option.
Upgrading the OS should be smooth but applications installed or third party programs on the server may have some issues after the upgrade.
It's also good to record the network settings before the upgrade, if the server network interface has multiple IP addresses.
Type: ipconfig /all > server_ip_record.txt to record IP settings.
If in case the upgrade messed up the IP Addresses, then it would be easy to put back the old settings.
If there is no spare machine or server for testing, be ready to troubleshoot any applications that might not function after the upgrade.
Cheers..hope it helps..