What is expected of a 255A? What knowledge would a 255A need to have?

Great question, CPT. It's really in the name for the MOS: Information Services. So that being said, a solid command of all things Windows (MS Exchange, Desktop Apps, O365, Azure, Windows Server, SQL, Windows OS, CMD, PowerShell, etc.) will help. Sharepoint will come in handy, you'll probably do a fair amount of that kind of work. Get comfortable working with VMWare if you're not already, most of your time will be spent working with VMs. Brushing up on your Cisco wouldn't hurt, you'll do CCNA in WOBC. Sec+ is required by DoD to be an SA, so it's good that you have that already. You'll do CISSP in WOBC (I think that's still in the curriculum) and with any kind of luck Army CIO/G6 will have free vouchers so you can go take the test. You will eventually apply all of the above skills to tactical networking, SATCOM and command and control systems, some of which you have probably already dealt with as a BN S6, but these are continuing to evolve even as I type this. Once you get to your unit, your new S6 will probably expect you to know how to perform simple desktop, laptop and printer repairs in addition to all of the other techno wizardry listed above, so it's a good thing you're already working at a Helpdesk. Great career choice and good luck, Sir!