Your weight loss will not affect a bike negitively in terms of the geometry. Your arms, legs and torso will still be the same length (unless you are a youngun' and still growing) after you loose the weight. What may change is the feel of the bike. Because you are heavy now the bike will flex more and feel a bit more squirly when riding. As you loose the weight, the bike will seem to stiffen up which will cause it to handle differently. Also, as the weight comes off you will be less likely to hit you belly with your kness Also, if you are like me and push the scales to their limit then 15 to 20 lbs lost will affect how a bike rides much more then if you are <200lbs.

Basically, if you find a bike you like now you will most likely like the ride when you have lost your weight. So I would say the answer to your question is No. Also, if you start saving now, plus what you intend to spend when you do get the bike you should be able to get a higher and machine

Good luck on the weight loss and enjoy your new bike when you get it.

Slainte