Hello @Iftekher ,
in general whether you change your contract or terminate it and order a new one, you should estimate that this process does not happen “over night” but require usually a few weeks. With a termination and a new contract you should also expect that this will not be a smooth transition and some time will be between the deactivation of the old contract and the activation of the new one. Changing the contract is the variant in which you will only have a few hours without internet on the day the move to the new contract is made, if that is of interest to you, then please let us know.
Kind regards, Sven