Once an application is created, the template assigned to a node cannot be modified via SDK and swis. But it can be accomplished at the time of creation of an application.
The powershell script provided in the samples (you will find it in the folder where SDK is installed) has information on creating, deleting application with other details of managing an application.
C:\Program Files (x86)\SolarWinds\Orion SDK\Samples\PowerShell\SAM.Application.ps1
Please let me know if you need more details.