Our system has ALIM connector creating the Work Area Folders in ProjectWise, all processing in PowerShell is to Work Area's that already exist.
I use GET-PWRichProjectTypes to get the list of all work area types that I can apply, and once the Work area type is applied to the Work Area (manually currently) then the properties can be updated UPDATE-PWRichProjectProperties, but I can't find a cmdlet to set Work Area Folder.
We currently do not have the ability to assign the project type. I am updating one of our cmdlets to allow this. In the meantime, you could convert the Work Area back to a folder (Convert-PWRichProjectToFolder) and then convert it back to a Work Area (Convert-PWFolderToRichProject) with the project type specified. This is a bit of a hack but should accomplish what you need.
I have added a few new cmdlets to get/set/remove an associated Work Area type. Look for these updates in a future release of the PWPS_DAB module.