  Digital Twins represent a new and disruptive technology, where digital
replicas of (cyber)-physical systems operate for long periods of time alongside
their (cyber)-physical counterparts, with enabled bi-directional communication
between them. However promising, the development of digital twins is a
non-trivial problem, since what can initially be adequate models may become
obsolete in time due to wear and tear of the physical components, accumulated
errors, or the evolving interaction with the environment. As such, there is a
clear need for mechanisms that support swapping in new models, as well changing
model structures as a whole when necessary. To address this challenge, we
propose in this paper a novel artefact, fmiSwap, that is FMI compliant and
allows for run-time swapping in standalone co-simulations, where different
strategies can be tested easily, as well in fully deployed DT settings with
hardware in the loop. We adopt a water-tank case-study consisting of a tank and
its controller to demonstrate how fmiSwap works and how it can support swaps in
a safe manner.
