My only cause for concern is that the shoe appearently moved in an unexpected way. If you can reason out why it moved as it did and everything is still OK with the piping, then there is not as much concern.

Extending the shoe is never a problem, I had a bunch of those happen on a job site where the shoes and pipe rack did not quite line up after installation. As more of the shoe is on the steel than off it, I don't think there is a concern. But if the field guys would feel better, I would not prevent them from doing it.

Basically, the only criteria for sliding displacement is that you need to be able to accommodate it in your design. I once worked on a main steam line at a power plant with a section that grew 14". You should have seen how big our slide plate was.

So my suggestion is make sure you understand why they pipe moved as it did and have a good idea of how you expect it to move in the future and then review the support.

Good Luck,
Duncan