I have create one Solution. on that time solution name was 'CrossSubsSiteAction'. deployed it successfully working my Custom Workflow Action from this articale : http://social.technet.microsoft.com/wiki/contents/articles/13604.creating-sharepoint-custom-workflow-activity-through-visual-studio-2010-en-us.aspx
after few days I have changes my Solution name and Project name to 'XYZ'. but my Namespace(Assembly) put as it is. i did not changes on Assembly. than check my SharePoint Designer workflow to list.
Error Comes up : Workflow failed to start.
Goto SharePoint Designer and and Publish workflow again its throwing error like.