a few days won't make a difference. stage 1 is the best as there's no sucrose while the subsequent stages, the milk tends to get sweeter. If cost is not a concern I personally will continue with stage 1 even after past 6 months, but too bad stage 1 milk is the most expensive. as such i will keep to stage 2, but never beyond that. I'd rather let my child drink cow's milk than stage 3.
I just moved onto stage 2 when the same happened to us
in my case, i did stage 1 past 6mo