If we change demographic targeting mid-campaign (when a campaign is in flight / running), the CPM will not change for an impressions-driven campaign, (This is where Awareness is the Marketing Objective)
However, for a Consideration/Conversion campaign (focused on clicks) it is set up as a CPC campaign, (based on Cost Per Click) so changing the demographic targeting would narrow the audience size, so we would need to redo the forecast/proposal for you, and we would ask you to sign a new IO in the platform under the same campaign to reflect these changes.