You can migrate a Mendix project by simply opening the project in the higher version and letting the automatic migration scripts do the work. Within a major version (7.12 to 7.13 for example) should always be easy and mostly without issues.
A major upgrade (7 to 8) potentially has more work associated with it.
Often it works best when you first upgrade to the highest Mx7 version available before going to 8 (preferably a lower version). Any issues that arise should show in the Error log or after some simple testing (styling incompatabilities are most common).