To play the devil's advocate, by that point the Mexican War veterans are going to be dead or retired. Their "institutional knowledge" of Mexico's dangers might be lost.
Well...not exactly true.
Joe Johnston lived until 1891 and was running all over the country inspecting Railroads for Grover Cleveland's administration and in the late 1870's/early 1880's he was a congressman. In a victorious Confederacy he may remain in the Army until his death, he might become Quartermaster General or Adjutant General or even General-in-Chief depending on his standing with the government. He'd be 73 in 1880 and while that'd be rather an old age to lead an army you have to remember that he was no Winfield Scott in his old age - fat and too ill to take the field - he was an active, workaholic who travelled all over the country right up until he caught pneumonia at Sherman's funeral.
EDIT: additionally, Johnston had been through northern Mexico with his brother-in-law in the 1850's to inspect possible routes in the event that the Union gave Benito Juarez regime recognition and military aide, so he was at least somewhat familiar with the area.
Beauregard lived until 1893 and was still living a relatively active life in the 1870's and 1880s as adjutant general of the Louisiana Militia and Commisoner of Public Works. He'd be 62 in 1880, still young enough to lead an army.
Edmund Kirby Smith lived until 1893 and was a university professor from 1875 to his death. In 1880 he would 56 which is a perfectly acceptable age to command an army considering Lee was 55 in 1862.
James Longstreet lived until 1904 and was a major general in New Orleans in the mid-1870's, policing the city and served as an ambassador to the Ottoman Empire in 1880. In 1880 he'd be 59.
Anyway, point is that there will concieveably be Confederate Generals who experianced the Mexican-American War first hand who could be in positions to either influence the campaigns or lead forces in the campaigns in Mexico should a war be fought in the late 1870's and early 1880s.
And the Confederacy in OTL were not bereft of talented young officers - some of who admittedly died during the Civil War - so some of them may rise to prominence in a Mexican war - I'm thinking the likes of John B. Gordon and, if they survived the Civil War, William Dorsey Pender and Patrick Cleburne.