You are supposed to file them within 30 days of the appointment.
But technically, nothing has happened to any of the ones I have been late in filing. I have filed them effective of the date of appointment.
If the Annual Return goes in with the details of the new director on it before the B10 is filed, then it will reject.
I once took over a file where there was one director registered. When I phoned the CRO, they asked me to go back to the 70's to correct information because the company had been appointing people and them leaving, not leaving, etc. It was a big mess. I had to go back to the start and do an audit trail to correct.