The history is NOT stored in the XMP files. The history is only stored in the catalog. Your only choice is to backup the catalogs and Raw files. You can also backup the XMP files for a double backup of your edits and metadata.
Sorry for my late reaction, I was out shooting as the weather was just too nice. I´m running LR 5.7 on Win 7. What I did was creating an export which gives me a copy of the .RAW file and a .XMP file. When I open the .XMP in Wordpad I can read it has stored the complete history of changes that I made in LR up until the moment of exporting. What I would like to do is to open the .RAW file + .XMP to pick up the changes that I´ve made and continue from a certain point to make it better. I can do it as long as my catalog is in order but I would also like to do this let´s say 10 years later when possibly my catalogue get corrupted or whatever. The .RAW file + the .XMP file do contain all the data needed to do this but I can´t find a simple function open .RAW + import associated .XMP to do so.