Q:

After upgrading to TFS2013 RTM, the search from the web interface returns "TF51535: Missing or unsupported field name." 

After upgrading to TFS2013 RTM, the search from the web interface returns "TF51535: Missing or unsupported field name."  It looks like the default fields to search changed. 
How do you change the work item fields that are searched on by default in TFS2013? 
What I have found so far:

It is searching on Title, Description, Repro Steps\
In TFS2010 you could follow the steps listed here to change the default (at least I assume... I didn't actually use that version)

A:

This is likely to happen for anyone that started using bugs in an older process template and has continued to use and upgrade the template to work with the newer versions. 
Microsoft is now searching on Description, Title, and "Repro Steps" by default.  The error happens because "Repro Steps" is not in any of the work item types.  This field maps to Microsoft. VSTS.TCM.ReproSteps the old similar field was Microsoft. VSTS.CMMI.StepsToReproduce
One way to fix the problem:

export the bug work item type
witadmin exportwitd /collection:CollectionName /p:projectName /n:Bug /f:"Bug.xml"
add the new field
FIELD name="Repro Steps" refname="Microsoft. VSTS.TCM.ReproSteps" type="HTML" 
import the bug work item type
witadmin importwitd /collection:CollectionName /p:projectName /n:Bug /f:"Bug.xml"

This allows the search to find the field but with these minimum steps that field won't have anything in it.  The search will effectively be on title and description only. 
There are two other solutions:
- Change what fields are being searched on to not include reprosteps (I haven't figured out how to do this yet)
- Do the above, copy all the old data into the new field, show the new field on the WI.  This would allow the search of the field as intended.