You will have to be more precise, i can´t understand what you want to do and why.
When a master (which device, doing what ???) tries to talk to a slave (which arduino you use ???) via I²C the slave address has to be known before the transmission.
You say that you want to listen for incoming data but you don´t know the slave address and it´s changing every transmit ???
I guess you explained yourself wrong, normally you don´t need to change the variables used by the twi class.