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So, managed to get past those errors by using `ReplaceOnChanges`  on those subnet groups.
```ReplaceOnChanges = new List&lt;string&gt; { "*" }```
But now I'm hitting the next error, `You cannot move DB instance development-postgresql5cf0ed8 to subnet group development-postgresql-subnetgroup-d532d2f. The specified DB subnet group and DB instance are in the same VPC. Choose a DB subnet group in different VPC than the specified DB instance and try again.`

So apparently you can only change the subnet group of an RDS instance if you are also moving it to a _different_ _VPC_. That's a bit unfortunate. :disappointed:

Worked around that by moving the instance to the default VPC from the AWS console. Then, the second time, `pulumi up` was successful. All is well :partying_face: