ISE express is simply license entitlement, unlike ISE PIC which is different software. You would simply purchase the ISE express upgrade to be entitled to utilize ISE in normal function capacity outside of the limited entitlement.
You would simply purchase the normal licensing after the ISE express upgrade and install the normal licensing.
This is all explained in the ordering guide
https://www.cisco.com/c/dam/en/us/products/collateral/security/identity-services-engine/guide_c07-656177.pdf
Since you're internal you should really be discussing with the PMs at http://cs.co/ise-pm
4.2 ISE Express License Cisco ISE Express is a bundle of 1 virtual ISE appliance and 150 Base licenses. © 2019 Cisco and/or its affiliates. All rights reserv ed. This document is Cisco Public Information. Page 29 Additional ISE endpoint licenses (Base, Plus and Apex) can be added to the existing 150 Base licenses via the normal a la carte process described in this ordering guide. The maximum number of Base, Plus or Apex licenses in an ISE Express deployment is 5000, meaning that ISE Express supports up to 5000 Base licenses, up to 5000 Base and Plus licenses, up to 5000 Base and Apex licenses or up to 5000 Base, Plus and Apex licenses. Please note that AnyConnect Apex licenses can be used in an ISE Express deployment as long as it has Apex licenses. Also note ISE Device Administration license is not supported w ith ISE Express. The virtual appliance included in ISE Express is for a single-site deployment only, and cannot participate in a larger ISE deployment nor can it be paired w ith another ISE appliance for high availability. Customers w ho w ould like to expand beyond the constraints of ISE Express (say, add additional ISE nodes, or go beyond 5000 sessions), have the opportunity to purchase the ISE Express Upgrade, to convert their ISE Express node to a ‘normal’ ISE base license. Please consider the timelines for such a purchase since the end of life of this upgrade license has also been announced.
