Open then rdl for the report in a text editor or xml editor. It's just an XML file.
Towards the top are the namespace definitions. You want the one beginning with: xmlns=
It also ends with /reportdefinition.
So as an example, you can try changing this:
On your report server, you can see the version used by your SSRS with this URL:
In BIDS, SSDT, you can change the properties for the project, select the project right click and select properties.
There is a property for TargetServerVersion that can be changed.
They won't always work but worth a shot since testing those is easy and it could save you from rewriting the report.
Hopefully you have a non-prod environment to test in which is the same version as production