"Substantial revisions" means making significant changes or modifications to something, typically implying a significant amount of work or alterations involved. Full definition
In a responsible peer review, the authors may be required to make substantial revisions to correct any flaws in the science or methodology before their paper is published. (firstthings.com)
Even if the rule is finalized without substantial revisions, there are real questions about whether it would stand up to litigation, throwing further uncertainty into the mix. (educationnext.org)
You want to make substantial revisions to your current living will and believe starting over will be easier. (rocketlawyer.com)