How do you change to a different cellular provider on the I-Pad?

Remove the SIM and insert one from the carrier you want to use...

simple as that? i mean in an iphone you cant do that... are the ipads prepared to change a sim just as that? then could i be used any ipad on any provider worldwide (with the obvios exclusions on service, velocity that exist on other country / data service provider)?

iPads are not locked to any particular carrier. You can use any compatible GSM carrier you like. So long as they support the iPad (it's a data only device).