Hi @Kyuzo88
Thank you for posting your query on Microsoft Q&A.
Unfortunately it is by design that its impossible to convert a standard group into a role assignable group, essentially it is to prevent unwanted escalation of user permissions, or group owners accidently gaining permission to grant admin permissions, we have a short note on it here -https://learn.microsoft.com/en-us/azure/active-directory/roles/groups-faq-troubleshooting#why-do-we-enforce-creating-a-new-group-for-assigning-it-to-role---
One possible workaround would be to create the new role assignable group and then use a PowerShell script to add all users from the previous group to the new group.
Do let me know if you have any further queries, I would be happy to help!
Kind Regards,
Donal