My loop has been flushed a number of times, not as part of an intended "service" but rather cos I tinker fairly often. While my loop is stable though, I do periodically check the Ph levels to ensure all is well. To date I've not had a problem and Ph has remained perfect - very impressed with the EK Premix I bought.
A friend of mine, who used a car anti-freeze mix in his loop "nothing will live in that" has had somewhat less luck. When we were pulling apart his loop to fit some new GPU's his coolant looked like someone had triggered a genesis device, there was so much stuff growing it in. The result of our work dislodged much of this life which proceeded to entirely block his CPU block. I did say that such a violent kick-back when turning the the pump off while filling wasn't normal!
Anyway, back on topic, I tend to just give the system a flush with de-ionised water after draining the coolant, before re-filling with my usual pre-mix. My loop is external so I can easily isoloate the blocks within the PC if need be, making draining and filling very easy. I've been running the loop about two years now, though the GPU's have been updated in that time. The external assembly however remains unchanged.
Hope it all goes well for you.
Scoob.
