Correct, you cannot mix licenses today, licensing is at the Org level and is either Enterprise or Advanced Security. Typically Adv Sec licenses are 2x the cost of the Ent license. Since you already paid for an deployed your Ent license, talk with your sales contact, there will be an "upgrade cost" so to speak, but in reality you'll be replacing the Ent licesne with the Adv Sec license.
Note that if it's been less than 30 days, you can simply RMA the license, work with your sales contact, you'll basically get refunded for the Ent license and re-purchase the Adv Sec license. If it's been more than 30 days, I do not believe Meraki Support can simply "convert" the license. So if it's 31+ days, one simple thing might be to purchase the shortest possible (1 year) Adv Sec license and apply it as a renewal, this will upgrade your Org from Ent to Adv Sec. I believe this will not only convert your Org to Adv Sec, but also bump out your license co-term date accordingly, so you'd still get credit for the Ent license you already purchased. You should be able to confirm this using the license calculator https://dashboard.meraki.com/manage/dashboard/license_calculator and apply one MX64W Ent as an "add devices" and then add one MX64W Adv Sec as a "renewal" to get a feel for how the co-term should work out based on the dates specific to your Dashboard Org.
As others have mentioned, work with Meraki Support when the time comes, to review the process and confirm what your new co-term date should be.