There are 3 rules for going to a different weight class:
Rule #1: Your fighter can only go up in weight, he cannot go down.
Rule #2: Your fighter's height will stay the same.
Rules #3: If your fighter goes up in weight he will be 5 pounds under the weight limit, and if your fighter goes up 2 weight classes he will be 10 pounds under the weight limit, etc.
Example:Your fighter is a 5'6 155 lbs. Lightweight. If he goes to WW he will be a 5'6 165 lbs Welterweight. If he goes to MW he will be a 5'6 175 lbs Middleweight. If he goes to LHW he will be a 5'6 190 lbs. Light-Heavyweight.