Is it possible to transfer stacks to another Pulumi account?

You should be able to export the state file and import it with the other account. See https://github.com/pulumi/pulumi/issues/8405 for an issue about making this simpler but also with an example commandline to do the export/import.

Great - thank you!

there is also stack transfer functionality in the UI of the service. navigate to the stack's settings (options), and select the transfer stack button towards the bottom of the page.