Can I switch from Serial to SoftwareSerial at runtime with the same baud?
I think you are missing what the baud rate is.  It is the speed at which the data is transmitted.   So the fact that your two devices are the same is not an issue.   It looks like from the code that your gps is connected to the hardware serial(pins 0,1 on uno) and the printer is on a software serial( pins 5,6)   If this is the case, there is no reason to turn off either of them, they can both work. In the case of using multiple SOFTWARE serial connections, only one can be 'listening' at any time, so you must switch between them. This is not the case in your code, as there is only one software serial connection.