Allow enhanced PINs for startup with Bitlocker

However, still only 0-9 characters are allowed

The setting you changed only applies to new BitLocker startup pins.

Existing drives that were protected by using standard startup PINs are not affected.

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Important

Not all computers support enhanced PIN characters in the preboot environment. It is strongly recommended that users perform a system check during the BitLocker setup to verify that enhanced PIN characters can be used.

Source BitLocker Group Policy Settings

What else can be done?

You can decrypt the drive and then encrypt it again, which will require you to set up another password.

Decrypting the volume means that BitLocker protection is removed from the computer and the drive is decrypted, which can be time-consuming. When you decrypt the volume, all of the information stored on that computer is decrypted.

If you decide to tu BitLocker back on, it will either use the TPM on that computer or it will require you to set up another password.

Source What is the difference between disabling BitLocker Drive Encryption and decrypting the volume?

BitLocker Group Policy Settings

This policy setting permits the use of enhanced PINs when you use an unlock method that includes a PIN.

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When enabled

All new BitLocker startup PINs that are set will be enhanced PINs. Existing drives that were protected by using standard startup PINs are not affected.

Source BitLocker Group Policy Settings