What happens when an artist is asked to make major changes to a proposal due to unforeseen circumstances, like natural disasters or major changes to a project site?

If the artist is asked to complete a substantial redesign or create a new proposal, then the artist should be compensated for that work.
If major changes to a project result in a significant delay of payment (more than 30 days), then artists should be paid interest or otherwise compensated for an agency’s failure to comply with the payment schedule.  Americans for the Arts suggests this be written into artist contracts, as it is standard practice for other suppliers and contractors.