If programming in C# .NET to get data from PI 2012, will I be able to use the latest AFSDK 2016 R2 or will I have to revert back to PISDK?
Not only may you use AFSDK 2016 R2, but you are strongly encouraged to use AFSDK over PISDK.
The latest AFSDK support PI Data Archive 3.4.380 or later to use RDA(Rich Data Access).
RDA means OSIsoft.AF.Data and OSIsoft.AF.PI namespace's calls like InterpolatedValues, PlotValues, RecordedValues, Summary/Summaries.
PI Data Archive 2012 = 3.4.390.16.
So you can safely use AFSDK 2016 R2 to access PI Data Archive 2012 as Rick Davin says.
Retrieving data ...