YES, you can access your previous saves. The game stores your save data at the following location: C:\Program Files (x86)\Steam\userdata\65462247\465200\remote

SaveGame.sav is your actually save file. The game also appears to make backup files so everytime you swap out your saves, it'll generate a new backup file, so there's extra redundancy.

Just go to that location and copy everything inside to wherever you want to store your completed data. Then you can reset your progress from the menu and play from a new file. When you want to switch back, just copy and paste your completed file back into the save folder and it's all back!

Achievements are tracked separately by Steam, and any other unlocks are carried in the save file.

The game itself can only have one save file loaded at one time, so just swap them in and out based on the one you want to play!

Enjoy!