Changing a data plan

[AshishKafle]

Actually I'm using a data plan which is 31£ per month for an unlimited data 

but there is a offer in sky plan 

for the new sim as 21£ per month for unlimited 

is it possible if I get a new sim for 21£ per month and stop the old sim 

and keep the same number of old sim to new one 

so that I can pay 21£ per month for unlimited data ? 
thanks 

[Daniel0210]

@AshishKafle 

That offer will be for a new sim with a new number. 

[PandJ2020]

There is a protracted workaround but requires you to port the number out of the network to port it back...