OP, like other FT-ers say you HAVE to refer to your specific fare rules. DL agents before reticketing will likely check the fare rules of your ticket, esp if the SDC engine doesn't allow them to rebook routings that are not allowed.

Use EF to check your specific fare rules, unfortunately gone are the days of transparency (red tail had them) when you could actually view your fare rules pre AND post purchase.

As for agents letting you do this out of fare rules, YMMV. I would say don't push your luck.