All the other advice given is very good. But, to go back to your original question, lowering the handlebars slightly should not require raising the seat. Seat height is really a matter of the relationship of you legs/feet/pedals. As I've gotten more flexible over the last year, I've gradually lowered my handlebars a good 2 cm. Haven't changed the seat at all. As others have said, just do it correctly so everything is snug and your steering isn't affected.