I do not speak for IK Multimedia but my view (from memory) is:
As you already had AT4 and purchased AT5 SE, all of your AT4 gear should also be available in AT5, along with the new AT5 features (new GUI, resizeable, drag and drop, 600 impulses per speaker etc).
AT5 would, on top of that give you the (28?) new gear models, including the 19 T-RackS-based rack FX.
You could try asking Sale Support if they would consider cancelling your AT5 SE licence and moving you to an AT5 licence, for a suitable amount. You could also sugget that that upgrade path becomes a permanent offering.
Please contact
Sales/Account Support, via here. There is a link at the top of the Search FAQs page to a Support form. Response time normally is within 2 working days (US (East Coast) Business hours, generally), but in the current circumstances please allow some more time.