To migrate from a Blueprint Interface:
Open the Blueprint that references the Blueprint Interface.
In the Blueprint Editor, click the Blueprint Props button in the toolbar to display the Blueprint's properties in the Details panel.
In the Details panel, under Interfaces, find the Blueprint Interface that you want to convert. Click the white "X" next to its name.
In the dialog box that appears, click Yes.
The functions from the Blueprint Interface are converted to regular Blueprint functions, as if you had clicked the Function button to add them. You can now work with them as you would with any other Blueprint functions.
Find in Blueprints
In the Blueprint Editor, you can use the "Find in Blueprints" feature (Find in Blueprints on the Edit menu, or Ctrl+F) to find all uses of your Interface Messages, and all implementations of the Blueprint Interface, which should make migrating easier.
While using "Find in Blueprints," you can find the Interface Messages you are searching for in all Blueprints simultaneously by deselecting the Find In Current Blueprint Only check box.