Hi Mike,
Actually, script files are not changed during upgrade and can be used with any TestComplete version as long as they do not use features specific to that version.
As for the rollback compatibility... Hm-m-m... I'm not sure it is possible to rollback VisualStudio 2010 project to VisualStudio 2003 or open Word 2013 file in Word 2000.
The axiom you mentioned is surely a good one, but I know two supplementary ones: 'Never work on the only copy but do backup first' and 'five backups stored separately are better than three ones'. ;)
