:"My query is that can some one opt S/44ad at any time as he is following S/44aa and had tax audit in earlier year."
Professionals as specified u/s. 44AA(1) cannot adopt section 44AD but from AY 2017-18 can be assessed u/s. 44ADA.
Other than the specified one are required to maintain books of accounts u/s. 44AA if crossed the specific turnover/profit limits.
Section 44AB specifically defines the tax audit requirement of the said business or professions when the turnover limit crosses 1 Cr & 50 Lakhs limit respectively. ( If the limit not crosssed, no tax audit mandatory, but maintanace of books is must.)
Section 44AD gives special advantage to small traders for non maintance of books of accounts (& tax audit) till the turnover limit of 2 Crs.; provided they declare minimum profits 6/8% for (banking/cash) transactions.
Till AY 2017-18, assessee were allowed to adopt any assessment method normal/presumptive as per their suitability without any hitch; the same can be seen with the said example; but now there is cap of succeeding five years from this year. Once the assessee declaring income under presumptive method, for any reason adopts normal assessment procedure; then he/she cannot opt for presumptive method for next five concecutive assessment years.