Hey Erik,
Annoying, right? Unfortunately, you can’t transfer credits to your old subscription, and some resources (like Managed Identities) just won’t move between subscriptions.
Your best options is keep using your old subscription and manually swap billing to the new one where possible. Recreate only non-movable resources and move the rest. Check Azure’s resource move support to see what can/can't be moved. Migrate via backup/restore some services (like VMs, databases) let you back up from one sub and restore in another. Azure loves making things complicatd!
Cheers,
Alex