Changing python version is not something supported currently in Slicer. Things are tested and set up for Python 3.9. There would be a bit of work to get all the correct hashes for the whl files (matching Python 3.8) of all the c extension python packages included in Slicer core.
In terms of effort spent, you are probably better off setting your external environment to be Python 3.9 with rclpy to match what Slicer uses.