First, no, the 2008 Rio was a whole new shape and the decision was not based on any issues with the 2007 Rio (Start Hull & shape with different graphics).
The new shape was to add some "glide" to the Rio S'M/L. The designers did this very effectively.
You might like the new longer narrower Rio L (at your weight) alot better, but do not make the upgrade based on the fact that you cannot tack your Start type 2007 Rio L.
Tacking is not something you are going to want to do very much longer. Soon you will be planing and jibing.
First you need a much bigger sail.
What wind speeds are you sailing in. A 5.0 is is a tiny "trainer sail" for a fellow your size in anything less that 20+ knots of wind.
You could easily use an 8.5 m2 in under 15 knots if you are using a harness.
The larger sail will help alot with sinking the nose.
Where do you run your mast foot..... put it all the way at the back of the slot.
Are you using the Rio L centerboard/daggerboard much?
Id not using the CB/DB then you are not longer "rig steering" and can d most of your turns as
flare jibes rather than tacks.
With the much larger rig, you will be planing and soon learn to "carve" at least part way around your
Also, if you change your tacking "technique" so that you never go forward of the mast foot (step over the leaned back mast rather than stepping forward and around the mast. Also, once you rake the rig all the way back going into a tack, you need to sheet the sail in to drive the board up into your tack, but do not bring the mast back up (like you do when you mover forward to go around the mast foot).
Bringing the mast back up during a tack just stops all the turning motion (along with your forward motion).
So, simply "step over" the leaned back mast BEHIND the mast foot in the area where your 2007 Rio has the most volume and stability.
Step your front foot over the leaned back mast until your are facing the rar of the board, then pivot and bring your new front foot over the leaned back mast.
Sweep the rig out to perpendicular to the board centerline and you will have completed a "fast tack" and be ready to sail off on the new tack, never having sunk the nose of your board.
Hope this helps,