Just be aware, 620 Will be limit to 256 in the DHCP server.
and yes,you may install 6.4.2.5(some bugs in cp image viewing) or 6.4.2.4 or any oher 6.4.2.X version you would like.
i advise you first to upgrade to 6.3.1.15 and then to 6.4.2.4 or 6.4.2.5 (no images under captive portal bug)
start with:
ArubaOS_6xx_6.3.1.15_48451
and then decide: (both versions supporting the 205)
ArubaOS_6xx_6.4.2.5_48774
ArubaOS_6xx_6.4.2.4_48122