I am having an issue with our internal autodiscover.

At the moment we have 2010 and 2013 in coexistance. Our autodiscover record in DNS points to the CAS array of Exchange 2010. If I do a test-webservicesconnectivity for a user in Exchange 2013, I get an Error 400 Bad Request reply. 

How can I fix this?

Hi snorri788,

Exchange 2013 doesn't use CAS Arrays as there is no MAPI support except via RPC/HTTP which doesn't use the CAS Array object in 2010 either.
Besides, Outlook client finds AutoDiscover also by using SCP first for internal access. If it not success, then turn to DNS.

For your question, please run following command to double check the result of Test-WebServicesConnectivity:

Test-WebServicesConnectivity -ClientAccessServer CAS01 AutoDiscoverServer CAS01

Note: please try to add a DNS record and point Autodiscover record to Exchange 2013 CAS.

Additional, please pay attention to below article about Step-by-Step Exchange 2007 to 2013 Migration, for your reference:
http://blogs.technet.com/b/meamcs/archive/2013/07/25/part-2-step-by-step-exchange-2007-to-2013-migration.aspx

Best Regards,
Allen Wang

I ran that command and this is the result:

[PS] C:\Windows\system32>Test-WebServicesConnectivity -ClientAccessServer CAS01 -AutoDiscoverServer CAS01
Creating a new session for implicit remoting of "Test-WebServicesConnectivity" command...
Parameter set cannot be resolved using the specified named parameters.
    + CategoryInfo          : InvalidArgument: (:) [Test-WebServicesConnectivity], ParameterBindingException
    + FullyQualifiedErrorId : AmbiguousParameterSet,Test-WebServicesConnectivity
    + PSComputerName        : shr1exmbx01.scottish.parliament.uk