Adding allegations to charge is called altering charge

Section 212(1) of Cr.P.C. provides that “The charge shall contain such particulars as to the time and place of the alleged offence, and the person (if any) against whom, or the thing (if any) in respect of which, it was committed, as are reasonably sufficient to give the accused notice of the matter with which he is charged.”

The purpose of giving details of the incident in the charge is to give sufficient notice to the accused of the nature of the matter with which he is charged and which he has to defend.

So, if the charge mentions a specific incident but a different incident is proved through the examination of witnesses, then usually the court may like to amend the charges under Section 216 Cr.P.C. If the accused has been misled and there is miscarriage of justice, then the conviction based on such charge can also be set aside by the higher courts.

However, if there are minor alterations in the details of the incident as seen from the examination of the witnesses and if it is shown that the accused properly understood what allegation / charge was being made against him, and if the accused correctly defends against minor alterations, then usually there may not be a need to formally alter the charge under Section 216 Cr.P.C.

Likewise, if certain additional (new) allegations are being made in the examination of witnesses, you can cross-examine them to show contradictions / absence vis-à-vis their earlier statements recorded by police. This will show that the witness is improving upon his or her statement.

So, ultimately, it all depends on the factual matrix of an individual case. No hard and fast rule can be laid down in this regard.