Which exam board do you do?
Bear in mind your spec is now completely different due to the changes, but I assume the it'd roughly going to be the same
The biggest difference I found with German AS was relevance and interest. At GCSE you seemed to do questions like "what's in my town" and stuff like that, however at AS the topics are, imo, much more interesting. For example entertainment and media, tourism, family (which sounds boring but it's stuff like gay marriage in Germany etc etc, quite interesting).
Also, grammar. The grammar content compared to GCSE is much much larger, as I found for GCSE you could get away with bare minimal knowledge as long as you checked your writings and speakings grammatically. At AS, since you have to write essays without being able to check them, it's much harder.
One piece of advice I will say right now is LEARN GENDERS TO WORDS. Seriously. I made the mistake of thinking "meh 1/3 chance I can correctly guess it'll be fine", but when things get more complex with cases and prepositions that take certain cases etc, you want to show off that you know this verb makes the noun dative.... But you can't remember the gender to show you know that. Seriously. Seemingly tiny rule, massive, impact that separates good from best.
Finally speakings are different too. You don't spend weeks writing and learning text, it's all improvised. Fear not you do get an A4 sheet to take in with you (assuming yours is the same as mine), and you choose the topic it's on so you will know roughly what your teacher will ask you. It sounds really daunting, but trust me you will be fine. The idea terrified me at first but when we came round to it and I was practicing it was great because I was like "damn I can actually spontaneously speak German". Cool tings