It seems to be oddly route specific without rhyme or reason. For example CVG-MDW allows CLE-MDW, but CVG-PHX did not allow CLE-PHX.
Hence how I set up the OP for DPs. Though perhaps my example is a one-off?
Could be. I haven't looked at / booked many routes to notice discrepancies like that.
Here are some DP's then.
SFO-LAX changed to OAK-LGB (was able to change between the above mentioned airports as well)
DEN-IAD changed to DEN-BWI (was able to change between the above mentioned airports as well)
OAK-BWI allowed changing both directions between any of the above mentioned airports.