What if you were to make a divider of plexiglass, all edges smoothed, the width of the tank and a little taller than the gravel bed? You could insert the plexiglass at one end of the tank, across the tank, leaving an area of about 8 to 10 inches from the end of the tank. Scoop out the gravel from that 8 to 10 inch area and put down the eco-complete. About five days later remove another portion of gravel and put eco-complete in its place, moving the plexiglass further and so on, and so forth, every five days or so, until you've reached the other side of the tank. Hopefully the process that I just typed makes sense.
Doing it this way is slower than replacing it all at once but you would avoid a mini-cycle and your fish wouldn't need to be removed from the tank. It worked for me