@ITST, Thanks for posting in Q&A.
From your description, I know you want to remotely autopilot existing device in Intune.
Based on my research, your understanding "convert all targeted devices to Autopilot meant the HWID would be submitted automatically for these existing devices is correct", you can just set convert all targeted devices to Autopilot Yes and all existing devices in Intune portal will automatically submit HWID and do Autopilot without uploading HWID manually.
Be aware the "Convert all targeted devices to Autopilot" only works for Azure AD joined and registered devices, not hybrid joined devices.
Also, please be sure your devices meet the requirements below using the setting of convert all targeted devices to Autopilot.
- Running a supported version of Windows
- Enrolled in a mobile device management (MDM) service such as Intune
- A corporate device that isn't already registered with Autopilot
https://video2.skills-academy.com/en-us/autopilot/automatic-registration#requirements
Or you can submit HWID using online when trigger oobe using command.
Non-official, just for reference.
Hope above information can help you.
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