Working on a System Center Configuration Manager (ConfigMgr) Server 2008 R2 to 2012 R2 upgrade on a Primary Site Server with a customer, I ran into an isuse worth sharing.
During the OS upgrade, the server IP address was lost and it was reset to DHCP. We entered the IP adress information, saw everything come online, and started our testing. Everything appeared to be working, except our OSD Task Seqeunces. Checking in the SMSTS.log we found that after the client PXE booted, it could not reach the Management Point (MP). We checked the following sites and found the MP was running.
That lead us to check this post from Henk Hoogendoorn.
That was the problem and the soltuion. Since the server was reset to DHCP during the upgrade, we needed to reinstall the boot image certificates through the ConfigMgr Console. After that, the OSD Task Sequence worked normally, without having to redistribute any content.
Keep an eye out for this as you upgrade your site servers.