Does upgrading Revit 2015 require upgrading to Revit Server 2015?

[Jroper]

Also, does a newer version of Revit Server still work with projects that are 2014 or below?

[Chris.Aquino · Accepted solution]

Each version of Revit will only work with the corresponding of Revit Server. Therefore, if you have projects in 2014 and 2015 you will need to have both Revit Server 2014 and 2015 installed.