Leaving a few lower buds on a harvested sativa, then putting it back under 18 hours of light, will usually bring it back into veg within a month. Sativas reveg more readily than most indicas, which is handy when a great pheno came from a single seed with no clone taken. The reveg walkthrough at https://hazesativaseeds.com explains the odd rounded leaves that appear first and why the plant needs a few weeks before it looks normal again. The second run tends to be bushier and quicker.