In the situation you describe, it is highly likely that each patient still in the trial would need to sign a new version informed consent that reflects the important changes made to the trial (i.e. length change in the trial given the circumstances).
In longer term run studies it is not uncommon for there to be 2 or even 3 versions of the informed consent signed and on file at the site. it is not uncommon for there to be several amendments to protocols do to study duration as we'll.
Its a pain for sites and monitors to track just like IND safety reports issued during long term studies are a pain to track and get acknowledgement by each sites IRB. Each sites IRB would have to approve of the changes prior to initiating the process of re-consenting with the new version. Oh the paperwork!
As always, the patient is allowed to continue by signing the revised consent or withdrawing of consent and exiting the study. The patient is allowed to do this at any point in the trial anyway for a number of different reasons and that is standard language in the designed consents.