Can I switch the RM126x LoRaWAN Class device on the fly?

The decision to switch between two LoRaWAN Class device is use-case specific and it needs to be initiated and proceed from both the end-device application layer and the LoRaWAN Network Server. It's good mentioning this process isn't managed at the LoRaWAN protocol level.

On the RM126x loaded with the AT Interface firmware, the Class device is configurable in the S Register 603 :

It is indeed possible to engage a Class change with AT Commands on the fly but once an S Register is changed, it must be followed by an AT&W to save the new configuration and then ATZ to reset the device. The Class device change will take effect once the RM126x will be reset so you'll have to re-join the LoRaWAN Network Server by issuing an AT+JOIN. It is advised to also engage in parallel the Class device change on the LoRaWAN Network Server and leverage the RM126x Join Sequence to apply the Class change on both sides.