If I remember correctly, those deals you usually see from some partners that Smarty works with (Uswitch, MoneySuperMarket etc) are typically for new customers signing up where you get an exclusive deal for more GBs than the standard plans offer .. and this kind of promotion usually lasts for 12 months.
If you want to switch over (without taking out another plan) then take a look at the 'Change plan' page and see if there are any upgrade offers - these plans can be switched to at any time. I had the option to switch my 16GB plan to a 32GB plan during the 2023 Black Friday without needing to open new accounts/ add another SIM etc. You can do this via the Smarty app also.
Having said that, when you click the promotion link and are redirected to the Smarty website I would advise you to check the T&Cs to see whether it applies to existing customers or only those signing up, just to be sure.
