I would clean it up, that is, cut off the worst of the yellow leaves, and trim the bottom leaves (helps with bug control also), and leave it at 400 for a couple of weeks, and then slowly raise it, keeping it around 600 in Veg.

Once in flowering, I try to keep the nute level at 600 to 800 for the first week or so, and then slowly raise it to around 1200 till its time to flush