Kind of hard to make recommendations since we don't know your personal taste.
My suggestion is to wear clothes that are comfortable and show off the efforts that you have made.
I wear athletic (meaning tighter) than what most people wear these days.
My jeans feel great... I wear almost exclusively Ralph Lauren polo shirts unless I'm working in the yard, then I go without or wear a T. Cargo shorts are great and professionally I wear a nice cotton pant, dress belt, Ralph Lauren custom fit polo (unless its a major client day, then its an athletic cut dress shirt & tie.
My suggestion is to make a couple of your new pieces of clothes nice ones...
like a RL polo or Abercrombie, some new shorts (needn't be expensive). The "transition" clothing idea (above) is good, but you can't put your life on hold forever and if your progress seems to slow down or stall, you want some clothing with will help reinforce "where you are going".
Good luck to you.... and congrats, fantastic you've come this far!