I currently use Libre Office Version 3.5.7 on Fedora 17. I am planning an up grade to Fedora 21 so am looking at my current files will be affected.
As long as you used odt, ods, opp,etc formats the files will be the same. No loss will occur. Should you have saved some files in doc, docx, xls, xlsx, etc formats, a difference can not be excluded.
If you want to test, use a portable version on USB stick and test your files. Download of portable versions can be done from here: http://www.libreoffice.org/download/portable-versions/
I have my first LibO files created in 3.4 and use now 4.3. No problem ever observed.
In respect to MSO formats, I stay always as long as possible in LibO native formats and save only the last saved version in an MSO format and try to abvoid the x-formats. I made the experience that a pdf-file is in most of the cases sufficient. Many people send out files in MSO or LibO or other formats when a pdf-file is sufficient. Depending on the situation you can even secure a pdf-file against copying and changees. The pdf-export function in LibO works very well.