Looks like the Odyssey-class is still its original length: https://twitter.com/DaveBlass/status/1647014461054541825
So do you think Shelby has always been in command or do you think she was just in command during the Frontier Day ceremony?
The latter I thought. If she's a Fleet Admiral she probably would have been a flag officer during the Enterprise-E's lifetime. Her last canonical reference was captain of the Sutherland during the Dominion War (Dax and Worf's wedding to be precise).
Looks like Shelby is a fleet admiral. Only in charge during the festivus .. But technically she finally does have command of a enterprise.