Hey there!
I've just finished my AS year french and I am definitely carrying it on at A2. Personally, I find it fairly enjoyable and is one of my more relaxed subjects (the others I take are chemistry and biology) which is why I'm not dropping it.
The big GCSE-to-A-Level transition is seemingly less significant for french than for other subjects (for me anyway). This is mainly due to the fact that in french what you mainly do is build up what you have touched on at GCSE. for instance, you must have come across le subjonctif at GCSE, correct? Maybe you have read it or heard it somewhere. (Don't worry if you haven't - you will develop it next year). Well at A-Level you are expected to use this at length, (unlike at GCSE where all you had to do was recognise it in a reading extract) as well as use other tenses such as le passé composé, le futur simple, and other things you have probably heard about, as well as a few new tenses.
Simply said, for grammar at AS, if you don't use a RANGE of tenses in your essays, you will get low marks, whereas at GCSE you could easily get an A* by showing a small range of simple present, past and future tenses. This is one of the major differences.
Another major difference is vocab (oh deary me). Trust me when I say you will HAVE to guess and improvise the meanings of at least half the vocab you come across in your exam. For your reading and writing exam at the end of the year, you will be given an article leaflet with what may seem like Greek. But don't panic. What I recommend is this: throughout the year keep a little notepad and note down EVERY word and definition that you don't know, which you have either heard your teacher saying, come across while reading your textbook, anything. Just make sure it is with you at all times. Even while watching that movie you have, note down words you don't recognise and look them up. My class and I had this and it really helped. This will not only help with your end-of-the-year essays, but also your oral test. While they prefer a more natural-flowing conversation, they also heavily praise sophistication. Before you go to bed your five-minute task will be to quick-read through this sacred notepad of yours. By the end of the year, you will not regret it!
Hmmm... What else? Oh, another recommendation would be to buy a grammar tense book. You can find these in WHSmith. They really will help you throughout the year, so that you don't get your tenses wrong.
So, sorry this turned out to be so long! Last-minute recommendations: don't just watch that one film! Fill your DVD collection with the french classics: La Haine, Coco before Chanel, The Chorus, Ma Vie en Rose. What you could also do is read french articles online as they will often give you articles to look at during the year. Optional would be an english-french dictionary. In hindsight I wish I had bought one.
Bonne chance!!