New plan

[TT_606016]

On currently 30GB per month at £10 pcm. If I switch to 100GB at £12pcm do I get a new SIM through post?

[Chalkychap]

If you do this via the online dashboard, I don't think so @TT_606016.

Have you ordered a new SMARTY SIM instead? 

[MSF]

@TT_606016 No, you don't need a new sim. When you buy the next plan, after the current one is finished, you can change to whatever alternative suits you - it will work with the same sim because it relates to what is enabled on your account really.