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Your overrides will remain once you upgrade but some of them my end up with outdated code as i'm sure there will be some files you have overridden that have changed between the 2 versions, 3.1 was a significant release .
As a guide on how to deal with this is if you get a list of the files in your override directory and use a tool like diff checker then compare the updated original in components/com_community/templates/default/ to the ones in the override directory yourtemplate/html/com_community and merge the files. unfortunately without using a version control system this can be a detailed task.
I hope this gives you a few pointers on how to proceed