Yes, you absolutely can swap the lines between front/rear, that holds true for all road hydro groups. Out of the box ST-RS685 used to come without quick release connectors, so you'll have to do something along these steps:
- Undo bar tape wrap from stem to levers.
- Unscrew hose furniture from levers.
- Screw hose furniture into the opposite levers.
- Wrap the tape back, check the brakes.
That said, step #3 might go wrong in many ways. It's usually not recommended to reuse hose furniture, especially between different levers because connection might leak, in which case you'd have to cut hoses, install new furniture and assemble everything again.
If some oil leaks, you'd have to bleed the brakes.
The hose length might also be insufficient, in which case you'll have to install new hoses (both BH-59 and BH-90 work with ST-RS685) and furniture (brake olive kit) and do a complete bleed.
With mineral oil, things might get very messy, so I suggest to at least remove brake pads and wheels in order to prevent contamination. Try no to work brake levers without pads/disc.
If you feel uncomfortable doing maintenance on hydraulic brakes, I suggest to ask a local bike shop for help. Even after numerous brake services, I still dread anything that has to do with brake fluid.