Will loading an old saved game lose any progress?

Solution 1:

No, they will not overwrite. The thing you want to do is find something unique, like a person, or an object to look at when you save, so you can remember where you were if you want to go back to a different save. Otherwise you will just be guessing.

Solution 2:

The answer is.... Drum Roll Please! (Patatatatatatatatatataaaa)... no. As long as you are careful to load the previous save and save a separate save file when you complete the alternate ending, and you don't overwrite your previous saves, it will not affect your current game whatsoever.

Solution 3:

The only thing it will affect is autosave files, as any autosaves will start to be overwritten with the content of the save you're playing on. Hard saves are completely unaffected.