I would totally recalculate though too for safety sake. There's so much going on when you're making soap that a simple divide by 2 or multiply by 2 can go wrong. (not that it's ever happened to me *cough*)
No you aren't missing anything, except the possibility of miscalculating (tho multiplying by 2 shouldn't be that hard). It's always best to run any change through the lye calculator just in case.