As a Beginner following the Bike Ride Tutorial, I want to be able to follow the steps required to switch from using Google Maps to using OpenStreet Maps, so that I can learn how to override an existing field type.

A rework of the maps used in the Beginner Tutorial is required, switching from the current implementation using Google Maps to the OpenStreet Maps project: http://wiki.openstreetmap.org/wiki/Main_Page

https://doc.ez.no/display/DEVELOPER/MapLocation+Field+Type

the usage of GMap to render a MapLocation field is implemented in https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Bundle/EzPublishCoreBundle/Resources/views/content_fields.html.twig#L280
and you can override that block in a custom template as (shortly) documented in https://doc.ez.no/display/DEVELOPER/Field+Type+template#FieldTypetemplate-Registeringyourtemplate
or here https://doc.ez.no/display/DEVELOPER/ez_render_field#ez_render_field-Overrideafieldtemplateblock