Would it have been possible for Emperor Francis Joseph to change the succession so that his granddaughter, Archduchess Elisabeth (Rudolph's daughter) could succeed him as an Empress Regnant?
Technically yes. In practise it is doubtable, if it would've worked. FJ himself didn't want female succession. It's said that this was the main reason of why he had nothing against her morganatic marriage to Prince Windisch-Graetz.
With this every attempt to change the laws would've been useless anyway as she was not suitable anymore. But Erzsi was never brought up as heiress anyway, her education alone was not made for a heiress.