Itr filing

Whether an assessee can file different ITR form in different financial year?

Suppose an assessee having some professional income during F.Y. 2011-12. So, he/she opt for ITR IV during that F.Y. But in F.Y. 2012-13 he/she having only salary income no other professional incomes. So,the assesse can file ITR I during F.Y. 2012-13 or he/she must opt for ITR IV because of prev F.Y.

Please clarify,

Thanks