Migrating property from bool to specific value in string

Hello,

I have a ContentType which contains a boolean to specify whether a background color should be set or not.

Now I would like to give the editors the option to choose the background color through a textfield.

Therefor I would like to migrate the boolean value from the old field to a specific hex color in the new field.

I've looked at MigrationStep but the two properties must be convertible between each other.

I don't like the option keeping the boolean field around on the ContentType, but hiding it for the editor.

Any suggestions would greatly be appreciated.