I recently purchased a PIX 535, it came with 6.3(5). My question is, can I upgrade to the 7.x/8.x software directly or do I need to purchase a license for the new software?
If you've recently purchased the Pix, then I would refer back to your supplier, they should have shipped the Pix with the most current software release pre-installed, which is now version 8, or at the very least shipped it with version 7 installed. You shouldn't be expected to purchase an upgrade license for a new product.
you need to make sure you have the proper smartnet contract for it (software update assurance , i think) - or purchase a one time upgrade license.
scroll to the bottom of that page.