A ticket with two identical sectors for the same passenger on different dates is neither a return nor an open jaw. Unless you were booking with an LCC which effectively only deals in standalone flights (and even here I think you would struggle to have such a change effected), what you are actually requesting is that a return is changed to two one-ways.

This can be done, but would require payment of the change fee and a fare recalculation to reflect two one-way fares for the same sector. The fare you paid would bear no relation to the journeys now actually requested. I suspect the changed sector would have to be issued on a new ticket number, though.