I agree with Chris. Most likely you have a module (or skin or other extension) installed that wants a more recent version of System.Web.Mvc. It almost certainly installed a binding redirect (which is what you see in the web config file).

That kept your old setup happy.

When you do the upgrade, it knows no better than to dump in the files it has available. It looks like its version of System.Web.Mvc is older. and the binding redirect does not find it.

Given that you are working on your development PC, I suggest that you start again. This time, take a copy of the bin folder before you start the upgrade. After the upgrade try replacing the system.web.mvc with one from the copy. You may have to try a few replacements before you are all clear. Document what you do so that you can repro it to upgrade the live site.

Always take a full backup (folder and database) before you attempt the live site upgrade.

I don't think this situation is ideal, but it's my best guess to get you going.

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Best wishes,
- Richard
Agile Development Consultant, Practitioner, and Trainer
www.dynamisys.co.uk

Success!

Ok, it wouldn't let me post the code in the previous reply due to the tags but it was
assemblyIdentity name="Newtonsoft.Json"
bindingRedirect oldVersion="0.0.0.0-7.0.0.0" newVersion="7.0.0.0"

It had a dependent assembly System.Web.Mvc with a binding redirect:
bindingRedirect oldVersion="0.0.0.0-5.2.3.0" newVersion="5.2.3.0"

After commenting out this code from web.config I uninstalled older unused modules and this removed the compilation errors and upgraded ok.

So now back to trying to create new modules without getting the Newtonsoft.Json error.