Yes, or at least to the same degree as with already flown travel.

Depending on the fare rules, you usually (but not always) have the option to attempt an even exchange (keep the fare and its rules) or to use historical fares in effect on the purchase date. You can also always throw out the entire thing and start fresh, using today's fares.

The only wrinkle is that with already-flown segments, your flexibility to "start fresh" is mildly limited. You can absolutely change the rest of the travel to anything you want (within ticket validity), but this will run the risk of needing the already-flown components to reprice as an unrestricted one-way, which for international travel is usually very expensive. The issue is basically that you can't "keep the price" for flying half of a round trip subject to various rules if you then change your mind and don't fly round-trip.

For example, if you fly half of an EWR-LHR round-trip, you can usually change to a different LHR-EWR for a nominal difference in fare or an even exchange. But if your original outbound required a stay of no more than 3 months in LHR, staying longer will reprice both directions to fares that permit this stay, and if you instead want to use the open coupon to fly LHR-FRA, then your outbound has to reprice to a (very expensive) EWR-LHR one-way.