Good day Tuff,

From a support perspective and to ensure that you make an informed decision, please read through the following link

for more information on how to determine what products and versions of products are supported and compatible with various versions of K2 for upgrading and installation:
https://help.k2.com/k2compatibilitysupportmatrix

I would also recommend that you engage with our K2 professional services where a K2 Consultant can provide you with customized advice and guidance that best suit your client environment and needs.

Kindly feel free to mark as “Accepted solution, kudo and/me too” if you find this information answered your question or leads to your answer.

Thank you and kind regards,

Dumisani

K2 will not accept any liability for any issues arising from actions taken in respect of the information provided by any forum member.

PYao is correct. From a K2 perspective, we do not distinguish whether your Windows server OS was upgraded or installed fresh.

I know there are many lively debates in the multiverse about whether or not in-place OS upgrades are a good thing. I hesitate to weight in on this, other than to say the old support mantra "test, test, test!" If you are on Windows Server 2012 or 2012 R2, and looking to upgrade to 5.3, K2 will support your K2 upgrade whether on new servers or upgraded OS. 

Best regards,

Gail