OO to my knowledge has always been backwards compatible. Remember DOCX is NOT ODS (OO's open source format.)I have just opened one of my OO v2.x with OO v3. File was first saved in 2006. I altered it, saved the alteration, closed the file and re-opened with with no problems.
Sometimes when I work on old files, which is often since I use them as templates, I get a warning that the file will be saved in the new format.