What you're looking to do is called revegging. I'm assuming you're doing this indoors because it's way too early to harvest outside - but if you are outside this won't work because the days are becoming shorter. To reveg a plant, harvest the bulk of it, leaving say 4 of the lowest branches with the popcorn buds and sugar leaves intact. You can cut the rest above those branches, and you don't need to have any fan leaves left, it will grow new ones. If you're growing in a medium that will allow it like soil or hydroton you probably want to trim the root mass down to something about the size of a softball - the plant doesn't need that size root mass anymore, and the roots may otherwise get out of hand as it grows again. Now repot or replace the medium if possible, stick it back under 20/4 or whatever you prefer to veg at and leave it there for 2-4 weeks or whatever your normal veg cycle is. If you normally go straight to flower give it at least 2 weeks for regrowth first though. Then flower as normal.
You should end up with another healthy plant. Be warned though the process that the plant goes through to reveg tends to create some mutant looking plants while it is reverting - 3 leaves instead of 5 is common, extra bushiness or other oddities. It won't effect yield or potency though, and by the time the buds are really taking off it should have gone back to looking pretty normal again.
Hope that helps.