There will be many issues with a reconstruct of DF. Not to say it can't be done, but issues will need to be addressed at what cost.
City of Austin owns Comal st. UT would have to purchase and negotiate ownership or redesign with the city.
The roads that surround DF currently will need to be adjusted and remain in some fashion for Fire and EMS access. You're not going to be able to delete them.
I'd bet $50.00 against $5.00 that there are a lot of utilities under Comal especially, much less the other roads. The utilities I'm guessing are, Sanitary Sewer, Storm Sewer, Domestic Water, Fire Water, Power, Telecommunications, and any unknow items from the beginning of time. All those would need to be relocated, and it will not be cheap if even possible.
I'm guessing other SEC stadiums with natural grass TURF, do not need a practice field? Certainly don't need a practice field with artificial TURF. I'd expect no issues growing grass as Dell Diamond is about same orientation and they have no issues. Plenty of sun in Texas for crying out loud. Hell may need shading.
The best I would guess Texas could do is either place stadium in a new location completely, or relocate all the current supporting buildings around DF and then you can expand a little. Relocation of road along left and center fields could be tucked closer to tennis courts, but need fire truck requirements. More options could arise if Comal could be pushed east and utilities along with it.