As the co-termination date is based upon the sum total of devices within the Organization, the extra value input by adding the new MX84 licences will be shared across all devices in the Dashboard.
Unfortunately, it is not possible for Support to convert MX80 licences to MX84 ones, as @AnythingHosted suggested. Remaining license value, for devices being upgraded, can usually be taken into account in the commercial arrangements for purchasing the new hw and license, but that is usually done, in advance, via the Meraki account team.