I can only speak for maths out of those subjects. I got a pretty high A* at GCSE and at first found A Level maths quite easy (compared to further maths haha), but after about a month or so it really does start getting harder and you need to keep on top of it all! I remember learning about transformations in Core maths and I got upset because I really struggled.
But after making notes on it and revisiting it every now and then, a couple of months before the exam I was looking back wondering why I found it hard.
I think it's quite a big jump, but sometimes exaggerated. There are going to be parts you find hard, but it's how (and how soon) you deal with it that matters. A couple of other people in my group left revising until too late, so they were constantly asking teachers how to do different topics a week before the exam.
So my tip is, if you start revising right from the beginning (even if it's just making sure you finish the exercise you did in class) the jump isn't huge!! But core 3 and 4 look like a bigger jump...
All the best for your results!!
Oh, and feel free to message me about anything! I've studied the following at AS:
- Maths (AQA) - c1, c2, d1
- Further Maths (AQA) - fp1, m1, d2
- History (Edexcel) - USA and GB in 20th century; warfare 1800-WW1
- Physics A (OCR)