Can allegations against an employee be changed during a disciplinary hearing?

When disciplining an employee, it is very important that the allegations (or charges) are correctly described.
What can an employer (or the manager) do when it (he) realises, during a disciplinary hearing, that the allegations should be changed so that they are more accurate? For example, the manager may realise the allegations are not properly described or the evidence being presented does not adequately fit the charges.
There is nothing stopping the manager from amending the allegations before the Chairperson makes a decision. But it is vitally important to think if the changes will significantly change things to the degree that the employee may have to re-prepare himself for the hearing.
If this is the case, then the hearing needs to be postponed in order give the employee reasonable and sufficient time to prepare accordingly to the changed allegations. Giving an employee sufficient time to prepare his/her defence against the allegations/charges is part of a fair process.