I believe I've discovered a bug or unintended behavior. The API is failing despite returning a SUCCESS code.

It has to do with reloading the drawing template while the drawing document is invisible.

The new template is saved (in Sheet properties) but it is not successfully reloaded, visually despite a SUCCESS code. If you re-open the drawing and manually click "RELOAD" -> Apply Changes then it updates visually. But not via API.

Making the document visible (while to macro runs) make the code run correctly.

Also during testing, it was creating some general instability and crashing SolidWorks, I believe this is due to the bug leaving SolidWorks in an unknown state.

Sub FixDrawingTemplate(filepath As String)
    swApp.DocumentVisible False, swDocDRAWING 'this will break this function. DO NOT ENABLE

	

	Dim fso As New Scripting.FileSystemObject
    Dim filename As String
    filename = fso.GetFileName(filepath)

    Dim extension As String
    extension = UCase(fso.GetExtensionName(filepath))
    
    Dim partnum As String
    partnum = fso.GetBaseName(filepath)
    
    Dim swModel As SldWorks.ModelDoc2
    Set swModel = swApp.OpenDoc6(filepath, swDocDRAWING, swOpenDocOptions_Silent, "", fileError, fileWarning)
    
    Dim swDrawingDoc As DrawingDoc
    Set swDrawingDoc = swModel
    If Not (swDrawingDoc Is Nothing) Then
    
        Dim swSheet As sheet
        Set swSheet = swDrawingDoc.GetCurrentSheet()
        
        Dim newTemplate As String
        newTemplate = "C:\\newtemplate.slddrt"
        Call swSheet.SetTemplateName(newTemplate)
        
        Dim result As swReloadTemplateResult_e
        result = swSheet.ReloadTemplate(False)
        
        Debug.Print "Reload sheet format for <" & swSheet.GetName & ">: " & GetReloadResult(result)
        Debug.Print swSheet.GetTemplateName

        Call swModel.Save
        swApp.CloseDoc swModel.GetTitle
    End If
    swApp.DocumentVisible True, swDocDRAWING
End Sub

Private Function GetReloadResult(ByVal result As swReloadTemplateResult_e) As String
    Select Case result
    Case swReloadTemplate_Success
        GetReloadResult = "Success"
    Case swReloadTemplate_UnknownError
        GetReloadResult = "FAIL - Unknown Error"
    Case swReloadTemplate_FileNotFound
        GetReloadResult = "FAIL - File Not Found"
    Case swReloadTemplate_CustomSheet
        GetReloadResult = "FAIL - Custom Sheet"
    Case swReloadTemplate_ViewOnly
        GetReloadResult = "FAIL - View Only"
    Case Else
        GetReloadResult = "FAIL - "
    End Select
End Function