Hello,
If the Odometer is set using a whole configuration push, it will likely lose the current trip data. Although I am not certain what exactly happens to the data collected before the cfg update, it will likely be inaccurate, like you said.
A quicker way would be to simply set the Odometer value using an SMS command rather than an entire cfg update (assuming you can communicate with your SIM cards with no complications):
setparam 11807:22000
Two spaces before the command are neccessary since they delimit an SMS login and password if they are set. The 22000 is the value you want to set the odometer to.
If you want the Odometer value of the device to reflect the actual odometer of the car as accurately as possible, set the measuring mode to OBD, since it measures using distance travelled without error codes + distance travelled with error codes. And send the SMS command to set the odometer when the car is not being driven. In that case, the device will receive the command after turning on and will start adding trip distance to the new base value.