Keep Windows Server 2008 "R1" supported as long as Microsoft does
I still strongly disagree with your decision to drop support for Windows Server 2008 first edition and for Windows Server 2012 first edition. These operating systems will still long be supported by Microsoft, Server 2008 until 2020-01-14 and Server 2012 until 2023-01-10.
Your customers have made significant investments in these systems and have for sure good reasons to keep them.
Unless there is a significant technical problem preventing Kerio Connect compatibility with these systems, which I don't see at all, I suggest you reintroduce their support again. Otherwise it really seems you don't care much for your customers.