backbone is provided by tm. Even when you install new maxis fibre broadband, someone from TM will also come to install the cable and BTU. That is the deal between tm and maxis. They provide the line to maxis until the BTU. That is why even when you change, TM still need to provide fibre until the BTU.

If BTU spoil then you need to get it replaced. But in this case, you will need to contact maxis which will contact tm which you will need to wait both of the party to be free to come over to your place.

Changing BTU unit directly should not work. Cause AFAIK, they need to register you line information into the BTU. The reason why it would work is if they use same standard password for all the configuration. Not sure in this part.

Good luck and keep us updated on how it goes.