Wireless network was originally designed for data on b/g, voice on a.
Voice has not yet been implemented, and have decided to move data to a, and may use Vocera on b/g. Should I create a new SSID which is enabled only on the a radios, and reconfigure all clients to use this (over 300 clients would need to be touched....but I would know which devices had been missed), OR should I enable the a radio on my existing SSID, allowing clients with an a radio to begin using it, which should help me determine which clients do not have a radios (although I'm not certain if clients will choose a over b/g). I would eventually disable b/g on my existing SSID (legacy clients would either be upgraded or reconfigured).
You really should use different SSIDs for voice and data. That way you can configure QoS.
As far as moving the clients over you could either just configure the SSID to use only 802.11a or you could use bandselect. With bandselect it will push the clients to 'a' but if the client can't support 'a' is will still allow it to join on 'b/g'.