Cak:
I don't think you can choose a category by yourself . If you are limited by runway or something else, you need to have approval from the authority to use a lower category. Category should be a permanent value, regarding of actual speed and actual weight.
Exactly. Approach category is determined in certification. If I am very light one day I cannot elect a lower category because Vref fits that lower category with today's landing weight. But, on the flip slide, if I elect to fly at a higher category speed I am then bound by the minimums for that higher category.