You can chop the top, then let the rest baste in the light for longer, but like some have said, that will cost you about 1-2 weeks more for what I see as a minimal gain in bud weight. Your smaller buds at the bottom will gain SOME weight, but I would rather just cut the whole thing down when the trichs tell me to. Put new plants in to flower, instead of keeping those small buds in there for 2 more weeks, and you will be 1-2 weeks closer to your next harvest.
As you get better at what you do, you will probably choose to prune your plants more carefully before flowering, so that you can minimize the under layer of buds (shaky stuff) and maximize the top layer with all the light. The top layer will end up being a better canopy, and you will enjoy trimming a LOT MORE!
Just cut off any offshoots that are too far down the plant to get sufficient light. Try doing this with half of your plants, and leave the other half the way you used to do it. After this experiment, you will never go back to all that duffy stuff at the bottom of the plant.