That is only part of the answer... many people were able to buy the RF7 upgrade for approx $50 US and it worked with the RF3.5 controller. Previously, KnifeEdge stated that they are moving ahead and if the old controller worked great, but they were not supporting it or testing it for functionality.
Since 7.5 appeared only a day or two ago, no one has said whether the new version works with the old controller or not.
My guess is that in the not too far distant future, it will not work. It is my opinion. The other aspect of the upgrade is that the newer software will probably not run on the same computer as you used for 3.5. The newer versions require substantially more computing horsepower to get a good result. On sale you can get RF7 and a new controller for around $160. Not a bad price for the product delivered.
You could also keep an eye out on fleebay, often old software is sold there.