I've taken clones from a flowering plant and re-vegged them. Do a search for 'monstercropping' - I think that's what the technique is informally called. I did it mainly to experiment and the results were interesting. It did add several weeks to veg because the clones had to get back into veg mode, and in the meantime they were putting out weird mutant leaves and generally looked less happy than my other plants. After they got back into veg though, they got super branchy and bushy.
I'd only do it if you need to save the genetics but don't have any in veg right now. I've never tried reveging after harvest, but I might go ahead and try it with this monster 6' tall Buddha's Sister that I have in flower now.