It looks like the Report Designer is forcing the Schema of my RDL to 2010?
So if I load a report in the Designer with a 2016 xmlns:
<Report xmlns="http://schemas.microsoft.com/sqlserver/reporting/2016/01/reportdefinition"
...and then I save the Report, it ends up with a 2010 xmlns:
<Report xmlns="http://schemas.microsoft.com/sqlserver/reporting/2010/01/reportdefinition" ...
... and that is breaking some of my SSRS compatibility, because the 2016 schema defines (among other things) a <ReportParametersLayout> section.