According to at least one source I found, this kind of amendment is legal in Pennsylvania (where it happened before), but may not be in Texas.
What's clear is that both major parties did make a good faith effort to comply with the law, but with the dates of the conventions, they were not able to.
So, who's at fault? Texas, for having too early a filing deadline? The major parties, for having late conventions?
I would argue that to take them off the ballot would be the fairest option, in principle, but in practice, it will merely create a major headache for election officials who would then have to tally millions of write-in votes for Obama and McCain.
Prediction: If the court even hears the complaint, they dismiss it because of the implications for the actual election.