find / -uid 30 actually didn't return "everything" but probably did return a lot of things that are uid 30 (i.e. /proc, /var/tmp, etc). To be more manageable, try a directory with mixed files and see the output. For example, if you know you have mixed files in the home directory, try find /home/username -uid 30 also you can use -gid for the gid's...
Once you figure out how to find them all, you can setup a for loop to chown them. Basically it's something like for i in *; chown user:group $i; done
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