As the Balrog flies

[Formendacil]

One point about Morgoth and his physical form...

Originally Posted by Of the Darkening of Valinor

Now Melkor came to Avathar and sought her out; and he put on again the form that he had worn as the tyrant of Utumno: a dark Lord, tall and terrible. In that form he remained ever after.

Now, I will grant this this does not necessitate that it was right then that Melkor was last about to change his form. It simply means that was the last time that he did so. Though there is no documentary evidence to back it up, I would take the statement that "much power had gone out from Morgoth" as an indication that he was, in fact, unable to change his form again.