It really depends on whether you wanna change to stage 2. If you buy the stage 2 formula, you will realise that some components like DHA and protein is different, stage 2 is less than stage 1. It is usually based on the assumption that by 6 mths, babies should have started on some form of solid food and thus get added nutrients from the solid food that they're eating.
I'm not sure if continuing stage 1 though your son is already on solid food would affect in any way but generally, even if nutrients are overdosed they usually dont't get absorbed and are passed out. But if you dont't change to stage 2, the stage 1 could also be more filling and thus decreases the interest in wanting solids. This paragraph is based on my own reasoning, not that I read it on the internet.
My boy is on NAN HA 2. For NAN HA 3, I am able to feed it from 10 mths onwards. But for my boy, he has not started on any protein solids yet due to his sensitive tummy, so I will continuing on on stage 2 and start on stage 3 only when he is starting on fish and tofu.
That's my take on the stage formula