Just looking at the fares DFW-BCN r/t
The cheap I fare seems to have a $300 change fee
If you needed to return early and I class is unavailable, the D fare would seem to be about $1900 more expensive r/t based on paying $2700, so would be looking at a fare difference of $950 on top of the $300. If D class is unavailable, the J fare if significantly higher