It been - ahem - a few years since I have used one, so the brain cells are a bit rusty.
(Also, while there is probably a few in the building, I don't want to mess with other peoples hardware).
Since its been a while, I re-RTFM'ed and it appears not possible.
It feels like it should be possible by the cards serial port, but by my reading, it only allows basic status reading. To change the IP address of the card, you use the serial port to put the card into bootP mode, and then use ioManager to set the actual IP address.
I could be wrong in all this....
Whats the use case for changing the IP address on the fly?