I spoke with a consultant from Microsoft. Their advice was:

• Update the assembly versions in the web.config file

• Remove assembly version specific information from the .aspx files

So, a register statement should look something like:

<%@ Register assembly="Microsoft.ReportViewer.WebForms" namespace="Microsoft.Reporting.WebForms" tagprefix="rsweb" %>

That is, the assembly version is not provided. It will defer to the version defined in the web.config file.