Presently, it is not possible to simply change the project lead of a next-gen project via the user interface.
Please provide a feature via the UI, the same way as the Classic project, where users can simply change the Project lead to another user or simply turn the assignment of issues created to unassigned.
Usually, when a next-gen project is created, there's a field called "assigneeType" during creation, this is auto set to "UNASSIGNED". However in some situation, this value can be set to "PROJECT_LEAD". which might cause unwanted assignment to a Project_Lead.
Using cURL, you can be able to change the Project Lead of a next-gen Project. Please generate an API token here
You will have to run this via a Terminal, enter the details of the new users by using the accountId, this can be gotten from https://<your-instance>.atlassian.net/people then search for the user to know the accountId which is in an alphanumeric String. Secondly insert the correct project id and project key on the Script prior to running it.