Well, I would say it's possible for you to request a re-encrypt from the other side of a matrix connection and I'm sure most matrix hangouts or secure sights have an agent in place that would spend the simple action and initiate the re-encrypt. The problem is that once the line is already tapped, this very request for a re-encrypt would also be susceptible to interception. Even if the other side re-encrypted on a fixed schedule, they would need to pass the encryption key along to you somehow and this data-packet would also be susceptible.
Like they say in Unwired, in 2065 the researcher Heinrich Andrews at Stuttgart University basically won the battle of Cryptanalysis over encryption theory. 2072 computer security has switched completely away from encryption as an effective form of security and is merely an annoyance. IC, Sec. Spiders, Alert Responses and network topology are the primary mechanisms by which a system is secured....it's like, why worry about someone tapping your matrix communications when their brains are probably already fried in their futile attempt to do so.
So if you have someone ready to sniff, if they beat the other end of the line in an initiative test, they could do an intercept action and then they could just block the request or let it pass through but pick up the encryption key.
Just remember that the only way they can tap your connection is if they have access to a node/s through which all of the connection is routed through......those being the originating node, the final node, or all of the routing nodes in between, which is pretty unfeasible except in places with very few devices.