depends.. try installing it, it may tell you to remove all other versions, or you must have at least version ?.?? .. How big is it? It might be the complete package in one install (which I think is the most likely).

It should be totally fine. I upgraded from 1 to 2 to 2.5 to 3 to 4, didn't have to go back to the previous version at any point.

its not actual upgrades, its full dvds. remove everything and install reason 5 from bottom. its all about the licenses, and if you have recieved a license for reason 5 it doesnt matter whether or not its an upgrade license or what not. that only matters if you try to sell your licenses. upgrades must be sold together with the previous licenses. hope it helps.