I have few work items (stories) already in Jazz. I want to add a new
custom attribute (say, "Time Remaining"). I could add new
custome attributes. But, they appear only for subsequent stories that
I create. They do not appear for existing stories. Is there a way to
have all exisiting stories to show this new attribute with some
default value?

You can upgrade existing work items (the feature is admittedly very hidden
at the moment):
1) Select the work items in the "Work Items" view
2) Right-click in the 'Type' column
3) Select 'Synchronize Attributes' from the context menu

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Regards,
Patrick
Jazz Work Item Team

I am using Jazz 0.6. I don't see a "Synchronize Attributes' context menu. I see other menu items like "Change Type, Find Duplicates,etc". Was this introduced in a later version of jazz?

Just for more info, To reach the work items view, I ran a query and the query results opened in work items view. The view had a Type column. I also tried Window->ShowViews->Work Items View and it also pointed to the same view. Please let me know if I can synchronize using my jazz version.