Can I change custom attributes in EO once user lost EO license ? (Hybrid env.)

Answer is yes, you can, however not to the any value you want. If your organization sync AD extensibleAttributes to EO, and use them for some advance filtering etc. could be problem once user lost EO license and some change in extensibleAttributes in AD happens (usually you do some cleanup of values, or you replace original value by some neutral which means user is not active etc. ). This change is not sync to EO anymore, because user is missing EO license (O365 license).
Result can looks like this: in AD extansibleAttribute1 = “” , in EO customAttribute1 = “Marketing”

This could be the problem when you use Dynamic Distribution Groups which have members based on customAttributes . User does not have license, and customAttributes still show in EO original values from AD from age of success AD sync.

There are some ways how to solve it. You coul add back EO license to the user and wait for sync which will change the value/’s in customAttributes in EO, or setup/cleanup values in AD in extensibleAttributes, wait for Azure AD sync (this action requires that user without EO license is still sync to Azure !) and setup same value in EO via powershell 🙂 , yes simple, but sometime can head may spin:)