Did you have a customized defined WBS code in the file that you used to convert to 2010? If you did, check to make sure that you still have the defined code: Project –> WBS –> define code. If the code defination is missing, add it back in. Then go to Project –> WBS –> Renumber and select to renumber the project or a selected group of tasks. The renumber is activated when a at least 1 line is defined for the code define for the WBS.