larger disk for a zfs pool

[Dick Hoogendijk]

OK, my freebsd system runs on ZFS boot. W/ solaris getting larger disks 
for a pool was quit easy. Simply replace one disk from a mirror for a 
larger one, wait for the resilvering and after this replace the second 
one for a larger disk and wait for the resilvering again. That's it. 
Been there, done that. But my feeling tells me it is not that simple for 
a FreeBSD zfs root system, or is it?