Conversion of old scheme to new scheme is mandatory?

No.

At present, Nov-2020 is the last attempt for CA Final old course students, unless extended further.

Till then you can give exams under old course and conversion to new course is at your discretion.

It will be mandatory to convert to new course after all the available attempts gets exhausted under the old syllabus.

But I hope that such situation doesn't arise for you. Best wishes for the preparations.