Converting an Existing Picklist to Use Global Picklist

We have option to do this in salesforce. This feature has been added as part of Summer 2017:

Promote Your Picklist Fields to Global Value Sets

Promote a single picklist field into a global value set by editing your picklist field and clicking Promote to Global Value Set.

A new global value set is created, and the promoted field now references it. You can also use the new global value set for other fields.

There are a few limitations to be aware of:

• You can only promote fields that have fewer than 1,000 values (both active and inactive).
• You can only promote restricted picklists. If you want to promote an unrestricted picklist, first convert it to a restricted picklist.
• You cannot promote a field to an existing global value set. But check out this Salesforce Lab App for some help.
• After a field is promoted, you can’t demote it back.

There is no way to convert existing pick list to use global pick list. While modifying any pick list on user interface we are not given any option to choose global pick list.

I explored the metadata options as well. I modified field metadata to use

<globalPicklist>GlobalPickListName</globalPicklist>

in place of

    <picklist>
.
.
.
    </picklist>

and deployed the object file. Result was I got an error saying operation is not allowed.

Considering this to be a good feature it would be good if we can raise an idea for same.