simcard for X10 on orange

Hi

my grandma is getting an upgrade to an X10 on her orange contract but as she's happy with her current phone she is letting me have the X10 only thing is orange are shipping it with a new sim.

I was wondering if anyone can let me know if the new sim will work ok in her current mobile, and what (if any) functionality will be lost using the X10 with a pay as you go sim card?

Thanks
Paul