how to update extid of a subproject in parent project after it has been synced to UI in the past.

I'm trying to change the extid of a subproject and hence I committed the change in kotlin code for both repos (parent and child). However the change is not syncing when I'm trying to upload new changes from VCS. I believe the parent has a extid for child in cache and it never dumpts it even if new changes are available in VCS root. Is there any way I can get this done.

Please note that I can't delete the parent project from UI since it has many other sub projects with history and hence deleting the parent project completely and recreating doesan't look like a good option for my requirement. 

I did try deleting the subproject in question from UI and tryig to resync again without any success.

Any suggestions?