Ntl modem broadband?

hi i have nth broadband, 10 mb and just want to know if i got hold of another ntl modem from e bay which is 250mb and attach that to my pc will it all work, but this time faster connection....or will ntl find out and restrict this from happening!! or will the upgraded modem of 250 mb have to be switched by ntl !! i dont know help plz.......... will i have 2 pay more then !!