This is from a video I prepared for the SAP Database and Technology Academy. Unfortunately, due to some personnel changes it hasn't been published there. I'll update the link when it has.
Currently only using the PowerBuilder.Net IDE, not available for Classic yet.
Must be using 12.5.1, the capability was added in that maintenance release.
You are actually compiling to either the x86 (32 bit) or AnyCPU (both 32 and 64 bit) target types.
If you aren't using ADO.Net for your database connections, you'll need to do so now.
If you are using System.Data.Odbc, then you need to make sure that you've created ODBC entries in the right area. There are two separate ODBC managers and two different areas in the registries for 32 bit and 64 bit ODBC entries.
Web Services and .Net Assemblies will take on the bitness of the process that calls them.