How does the CMRR of difference amplifier ICs change over time?
If the datasheet doesn't show the details of a particular contribution to the error of some value, then you have to assume that error contribution is already taken into account in the min/max range of the value.

In your example, the datasheet doesn't show CMRR drift over time.  You can therefore assume that this drift is already accounted for in the min CMRR spec.  That doesn't mean the CMRR won't drift over time, temperature, phase of the moon, or what you had for breakfast, but that it won't go below the specified minimum under all combinations of valid conditions.

Design your circuit to tolerate the minimum CMRR, and all should be OK.  Most of the time you will get better than that, but you can't rely on that.

Note that this also means performing calibration to initial conditions found at first powerup provides no guarantee.  In fact, it makes the worst case worse since this will add a offset to the error value.  For example, let's say a parameter is guaranteed to be from 95 to 105, with 100 being the nominal.  That means the device could at any time, for any reason, drift around between 95 and 105.  If it happens to be at 105 when you perform calibration, by having the external circuit subtract 5, you will get 90 when the device drifts to 95.

To perform calibration with any guarantees, the parameter you calibrate out has to be separated into initial accuracy and a long term drift components.  If the device you are trying to use isn't specified that way, either talk to the manufacturer to see if more detail might be available, or use a different part.