I'm no *nix guy, but that should work on all platforms.
You can't mix data files. So, if you have tasks downloaded with your new installation, you'll have to set No New Tasks and finish all tasks (upload and report them).
Then stop the BOINC client and copy/move the BOINC data directory from the old installation over the new one. Check access rights for all files or reinstall BOINC over the previous installation to let the install script do that.
After that, BOINC should process the old data files from where they left off.
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