Such Amendments are not to be allowed as they completely take away an admission already made judicially, and completely change the substratum of the case. In this regard you can rely on the following judgments :-
a) Usha Balashaheb Swami & Ors vs Kiran Appaso Swami & Ors-AIR 2007 SC 1663 - When leave to amend is sought to go back on previous admission – it shall not be allowed - as it may result in a material loss w.r.t merits of the case as he conducted himself vis-à-vis the proceedings in a certain manner owing to the Admission. Inconsistent Pleadings shall not be allowed to be made as are mutually destructive.
Another decision is :-
b) Revajeetu Builders & Developers Vs Narayanaswamy & Sons & Ors (2009) 10 SCC 84. In this case the court held that such amendments should not allowed where they are made belatedly with a mala fide intent just to prolong and protract the proceedings or where it changes the fundamental character of the case.
However since an amendment application is accompanied with an affidavit, perjury has already been done and you can after defeating their amendment application - initiate 340 CrpC.
Feel free to talk !
*Advocate Supreme Court of India