These are some very curious protein interactions. Quoting from the article:
"Why does the spike protein go through this many conformational changes to infect a cell? It "may be a way of the virus protecting itself from recognition by antibodies," Benton said. When the spike protein is in its closed states, it hides the site that binds with the receptor, maybe to avoid antibodies coming in and binding to that site instead, he said."
end quote
It seems unlikely that hiding from antibody responses is the reason for the conformational changes since, according to the article, the changes occur only after the spike has made contact with ACE2 and begins the changes to expose these new sites :
"They found that the spike protein undergoes shape changes as it binds to the ACE2 receptor. After the spike protein first binds, its structure becomes more open to allow for more binding..."
end quote
Since the changes appear to occur only after contact with ACE2, it seems unlikely that a vaccine for those "hidden" regions of the spike would be viable. Based on the article, such regions are not usually exposed, except during the conformational changes which occur during binding - likely too late to promote antibodies, or prevent infection. In any event, it is interesting, and spooky, that the spike undergoes such changes in order to deliver the viral genome.
Perhaps using a soluble, inactive ACE2 "mimic protein" would trick the spike into latching on to a "dead-end receptor", preventing it from entering a cell. Getting the spike to initiate these conformational changes before it can bind to the real receptor may severely limit its ability to infect cells and replicate. However, creating such a "mimic" may not be so simple. But if one could produce such a recombinant protein in large amounts, and it is safe, it could provide a cure if it is present for sufficient duration to eliminate an active infection.