Possible to stop "Read Only" after marking complete for Appointment?

Hello,

I was just wondering if there is any way to allow our users to edit their appointments after they marked complete?

If there are too many users in the "Required Attendees" party list and it is set as read only, the user is unable to scroll through all of the attendees and the field will not show all of the contacts. When it is an editable field, they are able to click on the field and scroll through the listed contacts.

I tried to make the field have more rows via formatting, but that option is not available.

Thanks!