upgrading from iPhoto to Photo; will my structure remain?

Not entirely. Events become albums and moments: Moments in Photos are the New Events.

Here are some user tips to help you with the migration from iPhoto to Photos:

Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac

External Editors in Photos Are Here

Photo Editing Extensions for Photos for Mac

All your albums will be transferred to Photos. Smart albums may be slightly changed.

You may want to have a look at this document: How Photos handles content and metadata from iPhoto and Aperture - Apple Support

Photo does not have events. Your iPhoto events will be migrated and saved as albums in a folder iPhoto Events.

ignore- my post has been posted twice