Thanks flyride.
If I understand this correctly the official answer is:
Backup your system before you try an update. And/or try the update first on a test system to see if it works.
If it works report it so others can learn. If it doesn't work report it so others know what is not working.
If it doesn't work you will need to find a new loader that is compatible with the newer update you are wanting to upgrade to.
In general it looks like security fixes should work (6.1.2 update x), minor releases are likely to work (6.1.3), and major releases are less likely to work (6.2).
New DSM version like a 7.0 are almost guaranteed not to work.
If I have anything wrong with my understanding please correct.