What happens when the Default Dashboard object is changed in Marketo Measure

Segmentation categories will be reset.
You can only set Segmentation for Lead or Contact, depending on what is set as the Default Dashboard object. The fields related to the object will be the criteria available when setting up your segmentation categories in Measure. (Opportunity segmentation is available regardless of the Default Dashboard object.)

Stage mapping on the Custom Model will be reset.
The Default Dashboard Object defines whether you pull from Lead or Contact for your funnel stages for the Custom Model. You can only pull stages from one object into settings.

Picking the Lead object will import all Lead statuses into the Stage Mapping page for potential use in the Custom Model. You can also create custom logic based on other Lead fields if you need to (Lead.MQL Date, for example).

Alternatively, you can pick Contact and all Contact statuses on the Stage Mapping page for potential use in the Custom Model. You can also create custom logic based on other Contact fields if you need to (Contact.MQL Date, for example).