If your online presence is integral to a successful business one of two things will happen if you chose to ignore the change.
1) Visitors looking for your business on the new .uk equivalent of your name will be met with a blank page on the internet. Some will correctly restyle in the .co.uk version whilst some will think the site is temporarily or permanently offline and head of elsewhere - maybe to a competitor.
2) Someone will register your equivalent .uk domain name and start trading either legitimately or in bad faith and take advantage of your presence on the UK top Internet platform.
Nominet have given .co.uk owners a period of time in which to register the .uk equivalent of their names. This still has sometime to run but at the point of writing this - a year of that time has already gone.
Ignoring this change really isn't a option if you want to stay on the UK's main trading platform. What we do say is that once upgraded, that is it. There will not be other upgrade in the future. You will be on the highest available domain name in the UK or anywhere else.