You can verify by yourself.
create first a simple ext with that permission in "permissions" section.
create a crx package with your .pem file
install the ext dragging the .crx file in chrome://extensions page
Then create a second version of that ext with that permission in "optional_permissions" section.
create a crx package with the same .pem file
install the new version dragging the .crx file in chrome://extensions page.
Now you can verify if you have keep that privilege from previous version or if the updating process has stolen it.
I would bet (no more than a penny) that old users keep the privilege already acquired.
Let us know