What is the ruling on exchanging shares of usurious banks for shares of Islamic banks, and is it obligatory to purify the money, and how is that done?

Contribution to usurious banks is forbidden, and the contributor must repent and purify his wealth. Since the bank does not allow withdrawal except by selling, selling usurious shares transfers the forbidden to another. The lesser of two evils is to remain in the bank while giving the forbidden wealth in charity. However, now that the usurious shares have been sold and shares in an Islamic bank have been purchased, only the principal and its profits earned after entering the Islamic bank are permissible. Anything beyond that must be disposed of by spending it on charitable causes.