Well, I'm disappointed, specifically took a day off tomorrow to go with my sister to the preview, as her pass is expiring soon.
No point in going tomorrow now. While I'm happy they're "fine tuning" the attraction, it's still lame that they postponed it, not everyone has the luxury of making or changing plans last second. They should've just sent an email saying "we look forward to seeing you during the preview tomorrow! However, please realize that we are still fine tuning the attraction, so it may not be 100% complete during your visit. However, we hope you enjoy previewing the attraction before it's complete for the general public!"
They could've easily allowed guests to ride it while still tuning it, and took advantage of this to make it seem more exclusive that we're getting to ride it before it's "done". Would've been better for PR.