If you have more than one Word version on a PC, they will share some resources and will view for control of the registry. In this case Word 2016 will always win, so unless you account for the fact that it is going to win, you need to ensure that Word 2007 does not reconfigure each time. You should also create separate templates and startup folders as it is not advisable to share the templates between the two versions.
The registry changes etc that you need to make are covered at
http://www.gmayor.com/Toolbars_in_word_2007.htm
Word 2016 needs to be the default version or it will simply change back whenever there is an office update.