Can you upgrade battlepass after purchase?

Pretty sure you can still buy it but you'll be paying full price for it. Also if I remember correctly it'll give you +25 levels no matter what your at. So even if your level 20 and you buy it you'll hit 45 on BP. At least that's how it was on season 1 I believe. Could be wrong, but since all the BP levels are the same amount of XP I don't see why not.