Unicode seems pretty backwards compatible; have any of the the newer versions overwritten or changed the meaning of older versions (e.g. caused damage)?
Yes. Version 2.0 completely changed the Hangul character set. Korean texts written with Unicode 1.1 were not readable in Unicode 2.0, and vice versa. This was 17 years ago, but note that it was after ISO had accepted version 1.1 as an ISO/IEC standard.