Hello!
I can't really speak personally on the jump from GCSE as I never did it at school, but I now do it at A-Level. Perhaps the fact that you don't need to have done it at school to take it at A-Level says something about the jump?
Most colleges recognise that there isn't really a massive correlation between GCSE and A-Level History to be honest. I have friends who got A*s at GCSE and failed this year whilst I never did it at all but have been getting top grades.
The main difference yes is the detail and volume of content, so anyone who thinks they can get away with reading over the textbook the night before an exam are frankly deluded. I think that's where a lot of people trip up.
It is really difficult, don't underestimate it. My advice would be to start early practicing exam papers, as it is just as much about learning how to apply your knowledge as it is learning the content. It is very content heavy, especially with the reform, as you will find that although an essay may ask about one part of your syllabus, there is a lot of overlap and it is very subjective.
As long as you revise thoroughly and keep on top of content I am sure you will be fine, but don't assume you have already aced it because of your GCSE grade. It really isn't an indication of the A-Level difficulty, if it was then they would require you to have the GCSE to do it.
Good luck! Any questions just ask.