Like any product with a 'use by' or 'expiry' date, once you go beyond it you can expect some kind of deterioration. But with e-liquid, it can be many months beyond it's expiry before any kind of noticeable change. If you continue to store e-liquid sensibly, you probably won't notice any difference at all - not within the first 6 months anyway. If changes do start to occur, this will likely be a change in colour or a slight deterioration in the flavour and strength of the nicotine.
But this can happen to any "in-date" vape liquid that isn't stored correctly.