The H2 Maths syllabus assumes knowledge of Additional Maths at 'O' level.However, schools may allow students who did not offer Additional Maths at ‘O’ level to offer H2 Maths if they show an aptitude for the subject. These students must be prepared to put in the work to bridge the knowledge gap.
Depending on the route you are going to take, either Poly or JC, A-Maths is going to affect you in some ways.