Is there a rule of thumb for when it’s enough to update the existing document, and when the design has changed enough to justify a new design doc and another review?
My rule of thumb is to ask myself, "Is there a chance my reviewers would not have signed off had this been in the design doc they reviewed?" If the answer is yes, I send it out for a follow-up and explain why I had to change the design.
In my experience, the response from my reviewers is generally, "Yeah, that's fine." It's a combination of (1) the practical limitations that it's hard for them to get the whole design back into mental context to argue about it and (2) they trust that I'm taking the design seriously and have thought this through. I think occasionally, I've sent a post-approval change out and someone points out something
It's common to encounter a curveball nobody anticipated at design time, but if you just go rogue and unilaterally make design decisions, it degrades trust and undermines the review process, so I want my reviewers to know that I'm taking their feedback seriously.