I dont know. I got really into maths in year 11, so my transition was super super smooth, with me knowing 90% of content in the course. It requires a lot more flexibility in some of the questions than what the gcse required, however, it's still got A LOT of procedural work, like all the integration etc. I found mechanics rather difficult, but I warmed up to it quickly, its basically phyiscs but just the maths part, and more focused on that (eg A level physics wont need differentiation, but mechanics might). Stats is a giant pain, its a lot of rote memorisation though!
Of course, the difficulty depends on the amount if work you put in blah blah blah, but rn, you could look into doing SMC, IMC questions in your spare time, and self teaching the course (very specifically for fun/challenging yourself)- thats what i did in yr 11 for a challenge around this time 2 months ago, and it seems to have paid off. Btw, you can look at GCSE FM and FMSQ rn