This is basically a question that applies for any game, but basically, what happened is that my disc for 'Payday 2' got scratched, and even after repairs, the game still cannot be read, and i am nervous about getting a disc in case the same happens again, so i hope to get it off the xbox live marketplace. My question is, will this new game keep the data that is saved to the hard drive, or will i have to get a new disc for this to work?
P.S. if so, will this be the same with other discs?
You can download it digitally and have your save be okay. The Xbox stores the save and the game installation as two separate pieces of data. As long as you leave your save alone, you will be fine. If you're really paranoid, copy it to the cloud before downloading the game.
Does your Xbox recognise the disc is Payday 2? If it does, you can simply install the game to HDD from a good copy of the disc (either from a friend, or rent the game).
This feature only needs the Xbox to recognise you have the game in the drive, once it has done that it will happily run the game from the HDD instead.
I used this method when I accidentally bumped my Xbox while playing Gears of War. Obviously if your disc is beyond all hope then you'll need to re-buy the game, but this method is a lot cheaper and definitely worth the try.
Yes, if you play a game on disc, and then start playing it on demand (as in, downloaded from the marketplace), it will use the same save file. It worked for me with Fable 3 and the original Crackdown; it will work.
Remember that if you bought it used you should have a unlimited time warranty (I could be wrong). So you can always exchange it for a brand new copy. If you bought it new you should have had I believe around 60-90 days to return it and get a new copy.
I would try backing it up to cloud storage suggested by the following sources. While it should let you continue from the saves from retail on the digital copy I cannot be 100% positive about that since it could be a case by case basis.
Just remember since its digital you will be downloading the whole game or a good amount of it to play it. File size is 4.56GB
If you really want to be able to test it before you do anything I would recommend putting it on cloud and going to a friend's house if you have one that owns the digital to see if your saves can work right off the cloud. If it does work than you got your A-ok in theory.
Sources:
http://www.gamefaqs.com/boards/707322-payday-2/67125583
https://support.xbox.com/en-US/games/xbox-games-on-windows/manage-games-data
