In terms of DevExpress you'll have to review their Breaking Changes backlog. It all depends on which controls you are using in your projects. If you can, wait a little while longer and move to version 14 which is due out in the near future.
https://www.devexpress.com/Support/WhatsNew/DXperience/files/13.2.9.bc.xml[
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As for .NET 3.5 to 4.0, there aren't a whole lot of breaking changes that I can remember. When you open your project created in VS2008 in VS2010, visual studio may perform a solution migration and will point out warnings and errors. Again, it all depends on what type of application and what controls and APIs are being used. For a list of changes from 3.5 to 4.0 follow this link:
http://msdn.microsoft.com/en-ca/library/vstudio/ee941656(v=vs.100).aspx[
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