I've talked to a lot of the people who have said they retroactively upgraded; and a month ago, it seemed pretty positive. Most people were being told "oh it's totally fine" when they called the centre.
In the past two weeks-ish, I've been seeing the second wave of people trying the same thing at the call centre. They're all getting mostly mixed reviews. Some say "yes", some say "no", some say "no" then "yes".
I tried calling a couple days ago, and they seemed to waffle about whether they do it or not.
It seems to be a gamble no matter what because Sheffield could just not honour it even if the call centre says that it is fine. But for those who did it successfully, their apps seem to be processed on par with their non-priority counterparts. (Some say quicker, but I haven't really seen anywhere that was the case.)
Ultimately, the decision is up to you whether to risk it or not. I think it's a noble choice to try or to stick with the application as is. However, I completely understand needing to be back. I wish you luck with your choice.
Edited for correct grammar.