You can but you may find that Jump and Maneuver values will have to change. T5 uses set sized 'engines' for Jump and Maneuver drives so you may find it hard to fit bigger drives in your ships than your used to under FF&S. At least thats what i found, but then i wasn't restricted to TL-9. The biggest problem i found was converting small 300dt and under ships with J-6 to T5, which it seems is now impossible if you want the ship to have any sort of function beyond being J-6.
Well, I don't mind at all just having to change some of the values around, as long as I can keep the same general concept of the vessel it will all be fine. The most prominent ship, and the one I'm most concerned about 'holding on to' is my Hammerhead-class scout ship. TL9 (often a mix of TL's, actually) Jump 1 (can make a variable number of jumps based on configuration), 1G (variable), 90 DT's (with a grapple for a 10 DT ship's boat), 2 9 DT modules spun around the hull for gravity (must be retracted and not spinning in order to manage a stable jump), and a 50 DT bay (most of the ship) for engineering that
technically is replaceable with other 50 DT bays (with different engines/jump drives/fuel bladders), but cost and time to do so are so prohibitive that no one except the military ever really uses this feature.
That was the ship that allowed the worlds of my pocket empire to reach out and re-establish contact with other worlds outside their cluster of stars. It's kind of important to my continuing storyline to be able to keep it. As long as I have that, I'm okay...even if it might need a bit of tweaking.
I've looked over some example of ship conversions from other rule sets into T5 (there's several examples of this on these forums - yay) and I think I will probably be okay. I may need to create some things that are unique to my setting, but I'm more than okay with that.
Overall, I think T5 will be both a blessing and a curse in terms of ship designs. The simplified system looks like it should take me less time to create a new ship which will motivate me to finish my shipyard, but I will lack the full control to fiddle with the details which I may find frustrating.
Apart from ship/vehicle/item design, I'm not too concerned about the rule differences. In fact, I am intrigued by the new task system. It seems quite interesting.