Reprovisioning nintex workflows across all site collections

I'm working on a fairly big application with many site collections. Each site collection has its own instance of workflows, which are being provisioned to it from the specified library. Upon modifying workflows, in order for them to appear in newly created site collections, I just need to upload them to this library in a place of the old ones, but how can I reprovision them to already existing site collections? I've planned to do it through PowerShell, I got a list of existing site collections with those workflows, but cannot find any useful commands that I could use to upload nintex workflow / .UDA file and publish it.