The down pipe is disintegrating into little white specs and clogging some of the hot side faucets.when I cut the hot side line to replace the water heater can I gently back flush the line with compressed air to unclog the lines?

My suggestion would be to connect a garden hose to the drain valve. Put the other end into a sink, a floor drain or outside. Open the drain valve and let the incoming cold water flush out the debris in the bottom of the water heater. Let the water run until clear.

If the drain valve is so clogged it will not drain, connect a clothes washer hose to the end of the garden hose and then connect to an outside faucet. Close the cold water input stop valve and open several hot water faucets. Turn on outside faucet and use the water from it to back flush the drain valve. Once the drain valve is cleared, turn off the water and disconnect the hose form the outside faucet. Close the hot water faucets and open the cold water stop valve and flush the water heater. Let the water run until clear.

Cut the cold water input pipe to the water heater and replace the dip tube (down pipe). At this time you should also replace the anode rod.

Remove and clean or replace all faucet aerators. Remove and clean any hot water valve stems that appear to be clogged.

Are you sure it is the fill pipe and not the sacrificial anode