Digital save file compatibility for Bloodborne
Solution 1:
The way that Bloodborne work with regards to DLC and save file is like so. If the save file was created on a game that has access to the Old Hunter's DLC, the data in the save file will be permanently altered to show that it was created on a game with said access. When the game tries to load a save file created in this way, it runs a check to see if the DLC is installed on the device. If it finds that the DLC is not present, it will give the message:
Unable to located Bloodborne: The Old Hunters DLC. This save data cannot be used until the DLC is downloaded again.
This message will always appear when trying to load the game without the DLC installed. However, this will not corrupt or in any way change the data in the save file.
If you do purchase The Old Hunters DLC and install it onto your device, then when the game runs the check on the save file, it ought to load it completely normal. It should load up the save as if the DLC was never uninstalled. No guarantee, but this this is how it should work.
For more information on someone in a similar situation, refer to this Reddit thread. Hope this helps and happy hunting!