Doesn't that just defeat the idea of streaming?
What's the point in going digital / streaming if I need a disc?
It has totally different directions if you purchased the digital version initially. Those directions which require the disk are just if you had the disk based version initially. The point is to prevent abuse/game sharing. Have you ever installed a game on your 360? Still need the disk in if you play it no? Same thing here.
If you are so interested in going digital, get the digital version the first time around. You can still upgrade.
Step 1: Buy or pre-order* the PS3™ digital version of the game from PlayStation®Store or Sony Entertainment Network Store.
Step 2: In PlayStation®Store or Sony Entertainment Network Store, find the PS4™ digital version of the game as soon as it becomes available. The special price will be available for accounts who purchased the PS3™ Digital version of the game from the PlayStation®Store.
Step 3: Buy or pre-order** the PS4™ digital version of the game at the special offer price. See Promotion Restrictions for Each Game.
*PS3™ games are available for purchase, and select PS3™ games are available for pre-order, from PlayStation®Store on your PS3™ system and Sony Entertainment Network Store on the internet.