Hi Aung,
there is no real risk when changing from 5V to 3.3V supply on ADS1148.
You should however make sure your input signals are still meeting the common-mode voltage requirement of the internal PGA. Also for your RTD measurement you should double check if the compliance voltage of the IDACs is still met. You might have to change the IDAC magnitude and/or the reference resistor value.
Regards,