There's always a chance that changing it could break something -- because of linux's bone headed design of storing filenames as opaque binary blobs with no defined encoding.
Basically, if some application created files with non-ascii characters in their names using ISO-8859-1 then those filenames will become garbage if you switch your system to UTF-8. To make the migration safely you have to first search your system for files with non-ascii chars in their names, change their names to be utf-8 encoded and then make the switch. IIRC there are some blogs that outline how to do that process.
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