Is there any way to figure out why a given

pulumi preview

is saying it will replace an instance?

In theory the properties after the

~

in the diff indicate which properties have changed. Running

pulumi preview —logtostderr -v=9 2> out.txt

and looking for

replaces=

can give more insight. There is a change in progress now which will provide more visibility into this by default. See https://github.com/pulumi/pulumi/issues/2453

is logtostderr new?

Not new - but a pretty low level debugging mechanism. Once we improve the diff reporting per the issue above this should t be necessary.