TLDR: pinned per instance.

When we compile a workflow into bytecode we get an executable; we store it and assign it an executable id. In-flight workflows are technically VM instances, and they always pin to a concrete executable id. When the VM goes out of residency (i.e. evicted from a node that executes it and unloaded from memory) we will eventually gc the executable bytecode from memory if it's not used by any VM for a while - but it's kept in postgres so VM can be revived.

There are ways to make workflows upgradeable - but don't have this yet. Ideally, we'd want to build it once we have enough feedback on how people want to do this - cause it's really about choosing the trade-offs more than anything else.