If you want a first hand experience of a transition bit by bit through the year then read this. I will try not to write a novel.
I am going to based this answer soley based on how I found the transition, which is the same as others and also different from others. Everyone will have a different transitioning experience. For referance, I took Law, Business, Psychology and Biology. I also stayed at my same school but we had a purpose built building made for my first year, so we had a completely different learning environment but the same people and teachers.
When I first started, I found all of my subjects very interesting. I had never taken Law and Psychology but had done GCSE Biology and Business. I really enjoyed biology GCSE and I was very good at business but didn't particularly love it.
For me, the first 3rd of the year or so, I didn't really feel like it was a 'jump'. I found it very different because it was a new place, different subjects and a completely different way of learning. After a few months I began to realise that my grades were averaging at an E and I averaged between A-C in GCSE. I also realised that I was really struggling with Biology, the work load was comparible to all of the coursework I did in GCSE combined (although Biology isn't a coursework subject). I also started noticing that the rest of my subjects were suffering as a result. I asked my teachers to let me off the course but they became very difficult to deal with and started kicking me out of 40% of my biology lessons. (Before you question my behaviour, my attitude and behaviour wasn't to blame at all, I have never ever been rude to a teacher before and even through this time was scoring A in my behaviour records. The reason they stopped letting me in was that I couldn't keep up with the workload, therefore they'd send me out to complete it during the lesson to catch up, then when I went back they'd moved on even more and that created a cycle of me having to learn most of the course by myself despite the fact I was struggling even with teachers help.) I virtually begged my teacher in charge of letting people drop subjects to let me out which was a long struggle but I eventually did it.
Once Biology was gone, it was like a massive weight had been lifted and I had a lot more space to do other work for other subjects. Quickly I started to notice that Biology had taken a massive chunck of time for so long that I had been slacking in my other subjects and found myself spending the next 3rd of the year playing catchup. (Despite this, don't think that this is a 1 off and won't apply to you, I would estimate that about a 3rd of students in my year done exactly the same thing after realising that 4 subjects was too much). By the time that I had completely caught up with everything and had gotten to where I needed to be, I was already half way through the revision part of the year and I also had insane amounts of paperwork, i'm talking 3 large folders that were bursting at the seems, 3 pencil cases and countless workbooks and text books that took up a very large tote bag and another large bag that I dragged from my car after tutor (the 2 were too heavy to carry at once). I also had a large locker that I couldn't fit all of the work into at any one time. At this point I hadn't even started making revision materials which would take up even more space so something had to be done.
My lessons finished most days at 3pm, I found that for about 2 weeks I would stay in our common area until around 5pm to go through the amounts of paperwork to reduce it, get rid of materials I no longer needed and file the materials into the right places. I was also doing this in my free periods. (over my entire timetable I had 50/50 lessons to free periods, don't let this fool you, even though you aren't in nearly as many lessons as GCSE, there is a lot more content which is harder and you'll need to use every single one of those frees to get half decent grades). Once this was done, I was able to fully throw myself into my classes and free periods and my grades made a noticable improvement. It takes a lot more time and effort to prepare for exam at AS than GCSE and it's something you'll just have to learn on your own. I done my exams with not to much worry.
To answer your question about the jump, when people say its a jump, it is but not in the way that I imagined the jump to be. Grades are different. I was achieving As and A*s in Biology all the way through GCSE and got to AS and fell flat on my face. Getting a C in AS is a similar level of difficulty to getting an A* in a subject that you're average at in GCSE.
Revision is different, you can't wing it, there is simply to much to know in too much detail so you have to spend a long time getting to know it all by heart. Therer is a LOT more revision to do. It is very memory based. Having all new specifications will make this harder - but the grade boundaries will be lower.
The layout of learning is different, you need to have a lot more self motivation, that motivation you have during the holidays and first week to do well and perserveer, KEEP THAT!!! You'll need it.
Exam preperation is harder and longer and has more detail, start early, revise as you go. My biggest tip is to make revision material as you go along while it is raw rather than wait until April to cram it in. It won't work, you need to start revising by January at the latest, follow this tip and BELEIVE. You will thank me.
Finally, plans will change, most of us walk into year 13 having dropped an option in year 12. Don't let that put you off taking 4 to start, it is a blessing to have the option to drop if you hate one, otherwise you're stuck. Don't be afraid to drop one early and if you do drop, do it as early as you possibly can and you'll have the best chance of catching up.
Good luck everyone. It sounds hard now but it grows with you like the difficulty has your entire luck. Have fun! Don't waste time! Don't spend more than 9 hours working a week or you'll fail at AS as you won't have time to breathe. (I'm not joking)