First, you don’t want to promote the scout to “adult” until his/her Eagle paperwork has been certified by BSA National and you have his/her Eagle credentials. Once (s)he’s an adult in the system, (s)he’s no longer a scout and updating any work he/she did as a scout becomes another level of difficult.
Otherwise, here’s what I learned: To “promote” a now 18 year old to Unit College Scouter Reserve (etc.) you have to remove them from the roster under “Manage Members” then under the same tab, “Add new member”. Type in their information, use the drop down to add new position then upload your file with their adult application, CBC and YPT