"Modifiable for a fee" means paying a (potentially quite large) change fee, plus any difference in fare between the fare you paid already, and the fares available on the day. Given that you would be changing to a flight that was just about to depart, it is highly likely that only the most-expensive one-way fares would be available, so you are probably going to be shocked at how expensive this would be.

I would venture that it might be cheaper to buy two one-ways now, and throw away the one you don't end up using.

My advice, though, would be to book the ticket that you know you can make, and not worry about trying to make an earlier flight.