The advice I was given when I trialled the Libre was that it does not replace finger pricks: it supplements them.
The huge benefit of the Libre is to detect trends.
I was told I should not use it for calculating insulin doses and it is not allowed to use to test BG before driving.
The Libre uses a different mechanism to calculate BG - for a start, it doesn't use blood; it uses interstitial fluid. This gives a reading which is, typically, 15 minutes behind finger pricks (from blood).
Some people find the Libre incredibly close to finger pricks or consistently out by the same amount so they can always add a certain amount to the Libre readings.
Others find it less accurate. Unfortunately, I am one of these people. This may be because my BG changes quickly when I eat or when I exercise.
I also find the readings become less reliable the longer I have the sensor in. The sensor is less likely to be able to provide a reading over time: I suspect this is because with all my exercise, the little wire sensor has started to move around even though the transmitter part is firmly attached to me.
So, to answer your question: I have never experienced a change in accuracy over time.
But you should not be relying on the Libre anyway.
The trends should remain pretty much the same.