My understanding is that the previous version was providing a safety valve to enable persons becoming automatic citizens through the amendments to the bill, but who did not wish to be citizens, to be able to opt out back to the time of the coming into force of the bill.
The significance of that is if they have another citizenship or reside in another jurisdiction where dual citizenship is problematic, they can be left with a gap in time, whereas this current version for the committee's consideration makes it clear that persons availing themselves of this opt-out provision would be deemed never to have become Canadian citizens under the provision of the bill, leaving no room for interpretation by another jurisdiction.
It is to eliminate or mitigate to the maximum any unintended consequences.