@smharch
Stable GUID is an unquestionable must, at least when working within the same BIM authoring platform...and it is (or should) not be affected by upgrading from one version to the other.
Since 2020 (due to complaints from some clients) we have been requesting all involved stakeholders to submit a validation report that includes all elements of the Structure tree with all GUIDs, both when upgrading models and/or when transferring models from one BIM authoring platform to another (when it is a must and for the avoidance of doubt)....Till date we have not recorded any anomalies or elements losing their Global identifier. (Edit: But again, this is a user platform and one can only speak of one's own experience. Your question is best confirmed by Autodesk themselves - tagging @Lance.Coffey ... maybe you can assist in confirming this)
In the past there as been reported incidents of lost GUIDs upon cross platform element copying (ex: IFC to RVT) but that is something which can be mitigated/avoided and I guess it is not related to upgrading models
Nonetheless, on the projects we work on the upgrade is neither encouraged nor recommend. We emphasize in both the CIR and the BES/P that models should be completed in the same version specified and were started in and archived to IFC with the IFC GUID stored in the elements.
