When Windows 10 comes out you will have to test. FMI usually does not test old products with new OS.
I doubt any official testing will be done for FM13, There is no money in going backwards. FMI is interested in their current and future products.
After 10 comes out you will see post on this forum if there are any issues. I would only upgrade a machine for testing purposes and test for a few months before I upgraded a machine I have to depend on.