Your question implies a problem that only sort of exists..
The question relates to how to get the trailer into a static state so you can unhitch, extend the tongue, rehitch, and back down.
2 problems exist when doing this...
1. When you unhitch, the trailer will want to roll down into the water uncontrollably.
2. When you unhitch the trailer might have negative tongue weight, and tilt(maybe)
Lots of trailer sailors have experienced this dilemma before you. 2. is pretty easy... you make sure the boat/trailer has POSITIVE TONGUE WEIGHT... you should do this before you leave your house (it's safer)... A trailer designed for the boat will do this when the boat is resting where it's supposed to anyway. But if you STILL by accident have negative tongue weight.... you can put a tilt-away jack on the back of the trailer, drop it before you unhitch.
For 1, you JUST need to keep the trailer from rolling away. SAFETY first! What I do is use a strap (tow strap, rated to 6 tons or more), I wrap it around the frame of the trailer, and back to the truck... THEN, I wheel chock the trailer. Unhitching is academic then. You can then see if the chocks will hold the trailer... if they do, remove the strap, relocate the truck, extend tongue rehitch.
The problems with our ramps are, the angles aren't consistent (5, 10, 15 degrees, so an extendable tongue would catch for me)... so I strap launch.
I self-launched my 25 footer (with 2 or 3 spotters for me)... so this works.
Good luck and go safe!