You can keep your plan depending upon which tab you want. You can keep your plan if you choose a small plan. You can keep your plan if it's $30 or more before add on's, taxes or overages, just your plan price alone. If you want a tab large though you can't keep your plan you need to choose a plan that makes you eligible for tab large.
You'll need a new SIM Card as the Galaxy S3 takes a micro SIM and your old Ace took a regular size SIM Card.
Just go to a kiosk or retail partner & they will be able to tell you your best option for the plan you have or if you want to change your plan if needed to get a bigger tab.