ReAssign a few Nintex Workflow Tasks

no, you have to delegate task o change a task assignee

depends on type of task action used ...

some task actions expose ActionID identifier, and you can let it save to a variable in action configuration dialog. for those tasks you can use delegate task workflow action.

however, some task actions (like eg. Flexi task) do not expose ActionID. for those tasks, not beeing task assignee, only way how to delegate it is to perform web service call, see SDK - https://help.nintex.com/en-US/sdks/sdk2013/#Reference/SOAP/NW_REF_SOAP_DelegateTask.htm%3FTocPath%3DNintex%2520Software%2520Development%2520Kit%7CNintex%2520Workflow%25202013%2520Software%2520Development%2520Kit%7CNintex%2520Workflow%25202013%2520SDK%2520Reference%7CWeb%2520Service%2520Reference%7CService%2520operations%7C_____7

note to delegate other user's task one has to have resp. workflow has to run with site admin privileges.

I'm using Flexi Tasks. I tried the web call service, it works with the list name. Is it possible to use the GUID (for example 675F51CD-8654-4CB4-85E1-BE4DAAB73112)? The reason behind this is, that I have many lists called Nintex Workflow Tasks and i need to specify it. 

You can use the list name.  When you specify the webservice call, you give it a URL and it will update the Nintex Workflow task list on site for that URL.  Each site will only have one Nintex Workflow Task list. So if there is multiples sites you will specify it for each site.

Thank you so much @emha  and @fccrous . It finally works how it should.

Have a nice day everyone :)