The new syllabus is the 2020 syllabus, not 2022, and it uses ECQB 2020 which has many thousands of new questions. The exams have been running for quite some time now so you certainly won't be the first to sit, however, I think the advice you have been given is based on these exams being less 'banky'.
When the old syllabus ends is really up to each CAA. It was supposed to be this month (January 2022) at the latest, but some CAAs have put in place extensions. I am not sure any CAA will let you start sitting on the old syllabus now though.
I am not sure any subject has changed that dramatically. Of course, communications is now one exam instead of two, things from WW2 like polar stereographic charts are gone, the question style is a bit different, and there is this new KSA module so there are some noticeable differences for candidates.