Can I install a disc version over a digital title of the same game?

I have the disc I didn't realise I still had a physical copy of the game. And I ou3chased a digital copy.

if you don't have the disk you can't play the game, so don't worry

It'll most likely just install anything you don't already have installed. The base game would be the same either way. But once it is installed you can continue playing as normal.

Specific to Xbox One, Digital allows for play without a disc present. It prohibits resale. Games purchased digitally can often times be refunded with a call to Live Support. I did this for the Xbox One version of the Destiny Digital Guardian Edition after
the news came out about the buy on 360, get it on Xbox One for free.

Disc-based games must have the disc in the console to be played. They can be resold. Disc-based games routinely go on sale unlike Digital, though MS has been better about sales on digital titles as of late.

There is no "size" difference as both digital and disc titles deposit the same bits on to the Xbox One HDD. In fact, installed-from-disc titles can be converted to digital titles (and thus obviate the need for the disc) by simply buying the digital version
of an installed-from-disc title. No re-downloading is required.

Hey! You must own a supported Xbox 360 title on disc or digitally. If you have a supported digital version of the game, you can install the game from the "Ready to Install" section of My games & apps. If you have a disc version of the game, simply insert
the disc into your Xbox One console, and the game will download from Xbox Live. You will need to keep the disc in the disc drive after the game is installed to play the game.

Here's a list of the supported titles so far:
http://www.xbox.com/backcompat