Upgrade to change box cast to assignment

[robe]

This is related to #2262 That takes care of new installs, but doesn't fix existing. I'm not sure how to change our machinery to drop and recreate a cast. As I recall I was able to drop the PostgreSQL box cast without any consequences so we could just do a

DROP CAST .. IF EXISTS;
CREATE CAST ;

I don't know how else to do it except my favorite DO call.

I suppose we could just always drop and recreate it too.

Anyway this isn't too big of a deal and doesn't need to be fixed for 2.1.0 (can wait for a micro). Only Bruce noticed the issue.

[robe]

Milestone renamed