Tested after looking at your post, it does work. I re-contract again for the same plan at the same price at $36.34 on 22 Oct 2023.So, just two weeks ago I was on the older 70 GB SIM Only Plus 45 plan, without the 20% off.
I checked out the upgrade to 120 GB without penalty, and just now I did another check out to the current 150 GB without penalty too! Today's the end of my billing cycle so I'll double check in a few days time, but yeah it looks like I've doubled my threshold at a cheaper price!
Either I have broke their billing/pricing engine, or Singtel is happy to just keep you/me/us on the same plan...
From 120GB to now 150GB and also have the email for $30 voucher for each line.