How to argue with postdoc hired for a project, to work on tasks from another project
You can switch her to another project if you have the funding for it. What you cannot do is to have her work paid by one H2020 project to carry out work for some other separate project. If that's what your postdoc says, she is right.
However, even in your existing project, there should be sufficient leeway to do work on good publications, research etc. and improve the work that you have carried out on the core project. Research never stops.
Alternatively, if you have funding for another project, then you can ask for the budgets to be reassigned (Edit: if she agrees) so that she is paid through the budget of the other project. That's perfectly acceptable, albeit you may have to return some of the budget of the original project she was hired for if that is not used in its entirety.
What you cannot do is to have the original project budget pay for work carried out on another project.