Have a look in the "game library" where you can re-download digital games you've purchased.

Putting the disc in the console does prompt it to install from (and always require) the disc, but when putting the registration code into the PS Store, it also gives you a digital download link.

I'd already deleted the trial and installed the disc, so I don't know if I could have saved a lot of re-downloading by never installing the disc version.

Having installed the digital version now, I never need the disc.