Reconnect to I2C with a different address (ATTiny85)
You only need to reinit the I2C slave.

The TinyWireS lib I found lets you just call TinyWireS.begin() with the new slave address. Though it doesn't have a way to stop the I2C connection. But the lib is hackable enough that with the ATTiny's specsheet you can add an end() to it.

However I believe that it may be better if the newly connected ATTiny acts as a master first and requests a free address from a known fixed slave acting as a dhcp server.