[Fr. Frank]

Peter,

1st, jack up the trailer in the back, at least 6" AFT of where you intend to move the axles. Do NOT let the wheels come off the ground. Put two automotive-type jackstands in place AFT of the jacking point, one on each side of the frame.
Next, jack up the trailer frame about 3'-4' FORWARD of the current axle placement. At this time, jack the trailer up so that all the tires come off the ground by about 1/2" to 1". Block-up or jackstand the two forward points as well. Try to do this so that your trailer frame is very nearly level, with the tires just off the ground.
At this point, your boat and trailer should be level, supported in four places, with the tires just off the ground.
Loosen the rolling carriage at all mounting points, so that the tires are now supporting all the un-sprung weight , and using the tires, with you and an assistant on each side, now ROLL the axles and carriage backwards to a pre-measured point AFT of your current point. 9 inches should be close to what you need. Make sure both sides move the same distance.
Now LIGHTLY tighten your carriage mounting bolts, then gently take the trailer down off the stands, and weigh for tongue weight. If you have the magical 5-7% of your gross weight, tighten everything up the rest of the way and your done.