Some things were never meant to work together. If you intend to upgrade some lights to LED, ensure that there are no old technology lights remaining on the same circuit.
LED drivers contain electronics that may be upset by old technology transformers on the same circuit. Some of the symptoms may include flickering and intermittent operation of the LED.
Given there are hundreds of different LED drivers in the market, symptoms may vary from serious to imperceptible. However, as with most issues concerning electronics, it’s the things you can’t see now that sometimes degrade slowly over time to manifest into a problem that you can definitely see later in time.