There are various Looper groups on the net that do this. Yes, it is against warranty but also i take the loopers' point that the medical device industry who manufacture these devices regulate how long the batteries last and this is an issue about profit at the individual's or supporting health system's (and thus taxpayers'') expense.
Diabetes is expensive enough without gouging by the medical device companies who are some of the richest health - related companies in the world.
I'd like to correct this statement - Looper groups
*GENERALLY DON'T CHANGE BATTERIES IN DEXCOM TRANSMITTERS*
It is not recommended.
Having said that, DIY users in general may re-battery old transmitters, either for use in countries where there is no Dexcom G5 or G6 availability or where lower cost options are required to maintain use, but I reiterate, this is DIY in general.
With all that said, it's probably better to replace the batteries in an old dexcom transmitter than it is to use a Miao Miao with Libre to make CGM, as the algorithm in Dexcom transmitters is designed to be CGM, and the calibration for MiaoMiao doesn't take in a number of factors related to the variations over the Libre's sensor life.