I know that the easiest way to convert your existing template to a new version of Civil 3D is to just open and save the template. However, you will be missing some of the new settings that just got implemented into the new version of the software.
One of these new settings can be found in the Drawing Settings dialog box, Units and Zone tab. There is a new "Imperial to Metric conversion" setting there. It defaults to "International Foot(1 Foot = 0.3048 Meters)" for the OOTB templates. This means that when you open and save your existing template, it will also be set to "International Foot".
So depending on your location and project, you might need to change this setting to "US Survey Foot(39.37 Inches per Meter)".
Although it could be found in previous versions, you may not know that there is a similar setting in the Land XML export settings under the Data Settings properties.