Tax Liabilty for unregistered period when the aggregate turnover crosses in the middle of the year

we are a huf unregistered under GST having business income and our turnover is less than 20 lakhs IF in november we rented our shop after which our expected turnover is crossing 25 lakhs
so my question is should i wait till my turnover crosses 20 lakhs or take my registration today itself And if i take the registration when turnover crosses 20 lakhs the dept can ask for GST of before registration period from april and sales and purchases records