Change of sides by advocate is not forbidden by Law, change of sides is forbidden if there are confidential communication by one side which may be used when the advocate represent the opposite party/side.

Take a particular case:
Confidential communication do not remain so which became public property during the conduct of the case and the engagement of a lawyer comes to an end with the termination of the proceedings of a case unless the lawyer is also retainer for the said client. Thus seeking discharge also becomes unnecessary. In such circumstances if the lawyer accepts a subsequent brief for the opposite party,he is not guilty of misconduct.

The prohibition or the bar of accepting a brief against your precious client is that there is likely hood of misuse of instructions though the misuse may not actually take place. The subsequent suit or proceedings can be said to be connected with the privious suit or proceedings of the former finds favour or is in consequence of the latter suit or proceedings. It is not the Identity of parties but the Identity of right or title.