Why not flush it? You might as well change all of the fluid right?
I have read the internet legend that a flush will somehow damage your transmission. My experience is with a Snap On transmission flushing machine, I have used it, and seen it used on hundreds of vehicles. I am not aware of a single one ever having a failure after a flush. Fluid flows through the trans as it would if it was running, no excessive pressures, no backwards flush. It is designed so that it can not possibly harm the transmission.
I think the root of this legend is so many people buying old vehicles with worn out automatic transmissions. They buy a car with a trans on its last legs, try a flush as a fix to a mechanical problem, the trans fails and they want to blame the flush. Fact is, the trans was cooked before the flush.
I have read the stories that claim sludge is knocked loose too gumming up the works. This is extremely unlikely as most auto transmissions are spotless inside. ATF is highly detergent, it is a closed system with no combustion taking place like in the engine. There just isn't the debris there is in an engine.
I think you should have it properly serviced with a complete fluid exchange and filter.