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Unfortunately, while it's incredibly convenient for the latest version, Homebrew makes installing previous versions of a package difficult. I'd try the instructions here: https://www.pulumi.com/docs/get-started/install/#installing-betas-and-previous-versions Can you provide more info about the issue you're facing in 3.38.0?

It fails adding a security group to a lambda function that needs access to a VPC

Diagnostics:
  pulumi:pulumi:Stack (lambdas-python-sgtest):
    error: update failed

  aws:lambda:Function (middleware-sgtest-WellsFargo):
    error: 1 error occurred:
    	* error creating Lambda Function (1): InvalidParameterValueException: Error occurred while DescribeSecurityGroups. EC2 Error Code: InvalidGroup.NotFound. EC2 Error Message: The security group 'sg-xxxxxxxxx' does not exist
    {
      RespMetadata: {
        StatusCode: 400,
        RequestID: "7f06c6f0-f439-4771-a660-f7c5ad964d11"
      },
      Message_: "Error occurred while DescribeSecurityGroups. EC2 Error Code: InvalidGroup.NotFound. EC2 Error Message: The security group 'sg-xxxxxxxxxxx' does not exist",
      Type: "User"
    }

I don't think that's an issue due to Pulumi CLI. That error is coming from the AWS API most likely.

It doesn't occur for a colleague running an earlier version of the pulumi cli

Are you sure you're both hitting the same AWS account with the same permission levels in the same region?

Now that I've uninstalled pulumi, I see that I rebuilt this project from scratch and managed to get one of the config params wrong. There are two aws region values and was only prompted for one of them for the stack config. The other got a default which is different and incorrect.

You're most welcome! Pretty common mistake - I make it myself still after using AWS for like 5 years.