Regenerating or re-vegging a marijuana plant is easy, and contrary to what was previously mentioned can yeild just as much as an un-revegged plant. You just have to be patient and be aware that stressing the plant can cause it to hermi.
I have successfully re-vegged both Power Plant ( from Nirvana ) and White Widow. After failing too get clones from my plants (long story)I decided to try re-vegging.
What I did:
After harvesting the major buds I left the bottom 1/3 of the plant and cut off the top 2/3. I left the small popcorn buds on the plant as well as all the vegitation on the bottom 1/3.
I trimmed about 1/2 too 2/3 off of the root ball and re-potted in fresh soil.
fed the plants with a high N nutriant.
Thats basically it. It takes about 6 weeks or so for the plant to switch back to veg and start showing signs of re-growth. The new growth will first be seen at the small bud sites, where new leaves will begin showing. The re-vegged plant will be as healthy and produce just as much ( or more) than it did befor.
I grow under 400 watt, in soil ( foxfarm) , for my indoor grow. And have used re-vegging any time I need to preserve a strain which for some reason I didn't get clones. It is an excellent way to preserve that 'special' keeper.
Good luck and happy growing.