LEO satellite handoff is tough because you effectively need two antennas: One points at the "old" satellite while one points at the "new" one. You can achieve this with a single phased array antenna divided in half and controlled by software, but each half must still be large enough to give the needed S/N ratio.
Iridium uses an omnidirectional antenna which is fine if all you want is 1200bps. But if you want 5Mbps, you have to point the antenna. So Iridium is not applicable here.
Cell hand-offs don't happen nearly as frequently as they would for an LEO constellation.
The orbital period for a satellite at 550 km is ~01h 36m. That means any given satellite is going to be visible for a very short period of time, which means hand-offs will have to be extremely frequent.
That's per satellite; each satellite is going to have multiple beams, so you'll have to do a beam handover like every 15 seconds and a satellite handover every couple of minutes
I don't think you need to move the antenna, but you need a wide footprint and probably overlapping coverage between satellites.
How does Iridium work? I'd think this has already been done(quite a while back even).