Gstr 1 monthly or quarterly

Hello, As per the old and recent changes the GSTR 1 filling is categorized into 2 heads: 1. above 1.5 cr "AGREGATE ANNUAL TURNOVER" and 2. below 1.5 cr "AGREEGATE ANNUAL TURNOVER"

what i dont understand is Turnover of previous year is to be taken or of 1st quarter of Current year?

if of 1st quarter than do we need to take average of 1.5 cr i.e. 12.50 lakh monthly average basis?

and if in worst case the aggregate is to be calculated in continuity than is there a possibility of quaterly changing to monthly from middle of the year?

totally confused

Thanks for the Help