The Futura 133 has enough volume to float your 200 lbs. and an 8.5 rig, but right at first it's going to seem like it does not have sufficient volume.... specially since you are sailing primarily in fresh water and will not have the additional bouyancy that salt/sea water provides.
The length of the board is going to give you more problems... right at first...with your uphauling vs what you doing on the Rio M.
My guess (I weighed 10-15 lbs less than your 200 lbs.) is that you will need to place your front foot just in front of the mast foot (use the circle printed on the board as your baseline for mast foot position.... I've found it to be quite accurate on where the "sweet spot" actually is) and your back foot a shoulders width
behind your front foot.
If you try to space your feet equally (as you probably did on the Rio M) you will find that the nose sinks pretty quickly.
So, you need to place your feet so that your weight is centered over the fore and aft max. bouyancy point,
which is not going to be where the mast foot is. Your weight will be offset toward the back of the board to put you in the more floaty, wider, thicker area behind the mast foot.
You will need to be a little careful and pick your conditions until you learn to waterstart, as in higher winds and when you are sailing in the ocean with swells and chop, you may find yourself in a situation where you cannot make uphauling work.
I had this expereince many years ago, and I learned to do a slow and ugly version of a waterstart that day.
Only way I could get up and sailing to get back to shore, so there were no options... it was siply too rough to uphaul.
So, start out in miler conditions with your 8.5 m2 rig and expect to fall off a few times.
Side to side balance and flotation will be less an issue and your skills learned on the Rio M will serve you well on the Futura 133.
Keeping you weight over the fore and aft centerline will be similar to what you did on the Rio.
Another issue you will find different is how quickly you can move back on the board.
The Futura will not want to plane real easily if your weight remains forward of the front footstraps so you
will definitely need to move back on the board, but you cannot move back too quickly or the tail will sink and you will loose all your speed.
PROGRESSIVELY is the answer here. Move back a little... gain a little more speed, move back some more,,, gain more speed.. do this in stages and you will soon learn how fast you can move back (relative
to how quickly the board accelerates) to keep the board accelerating without going too far and killing your speed.
Good luck... do not get discouraged... it will come together and you will soon be zooming over the water's surface at significantly greater speed than on your Rio M .. on a board that you can basically turn on a dime by comparison.