You can add the PS5 digital version to your current library now if you own the PS4 game. Also works if you get the game through PS+ in 2018.
From the game page:
If you already own the PS4™ version of this game, you can get the PS5™ digital version at no extra cost and you do not need to purchase this product. Owners of a PS4™ disc copy must insert it into the PS5™ every time they want to download or play the PS5™ digital version. PS4™ game disc owners who buy the PS5™ Digital Edition disc-free console will not be able to get the PS5™ version at no extra cost.
Thanks for this. I've redeemed it bit can't do the same for DLC that I own on the PS4. For example, checking the Halloween PS4 & PS5 DLC, it tells me it is unable to be purchased because I already own Halloween DLC for PS4. I hope when I finally get a PS5 and boot up Dead By Daylight it will just recognise the DLC and transfer everything over!