Roy, a former close aide to Chief Minister Mamata Banerjee, joined the BJP last week in Delhi.
"The change we fought for, has not been achieved. But the people want a change and BJP is the alternative", he told reporters at the party's state office.
Sitting beside state BJP president Dilip Ghosh, Roy said "We will fight under Ghosh's captain-ship in the state to bring the much-awaited change. Change is inevitable in Bengal."
It would also come to power in Odisha and Bengal, said Roy who arrived here amid a rousing welcome by BJP workers.
On how he would fight the TMC, in which he was once second-in-command, Roy said, "In the Mahabharata, we saw how the Pandavas had fought against their own brothers, the Kauravas".
He, however, said that he will make his political statement during the BJP's scheduled rally here on November 10.