I am taking AS music next year (edexcel)... but I am taking it 2 years early which means if I want to take A2 at any time, I will either have to do it at the same time as my GCSEs or else re-sit AS, which seems pointless.
Should I change my mind and not take the AS??

I'd sit the AS now, and then sit the A2 when you would have done the AS. That way you can do an extra subject in your Upper Sixth year, but don't have to stress out doing 11 desperately irritating subject exams, while doing an A2, which is about 3 times the workload.

Yes, it's good if possible to have even just one A level grade before you apply to university in the first place: they like to see that you've already done it and are more likely to give you a place; a real grade is more reliable than a prediction! If you're intelligent enough to be doing the A level early you'll probably find the GCSEs quite easy anyway : you could take two years to do the A2 part if you like so you wouldn't have to do the exam at the same time as GCSEs.