To fully answer your questions about joining another GLO, we'd need to know (a) if this is an NPC sorority and (b) if you've been initiated. Like KLPDaisy said, if you've been initiated into an NPC sorority, you can never join another NPC sorority even if you've disaffiliated from the first one. If you haven't been initiated, you can depledge and join another NPC sorority. You have to wait one year from the date you pledged - not the date you depledged.
You can join a service sorority or fraternity like Alpha Phi Omega, a non-collegiate group like Beta Sigma Phi, or a professional GLO at any time. You can even have concurrent memberships in one (or more) of these GLOs and an NPC sorority. There are also local, regional, and national sororities that are not part of NPC or NPHC, that you can look into.
Depledging and disaffiliating are fairly straightforward. There's most likely some additional paperwork associated with disaffiliating if you've been initiated. Your colony's advisor can tell you exactly what you need to do. You would be asked to turn in your new member/pledge pin or your badge. You would no longer be entitled to wear letters - you can't be forced to turn them in, but why would you want them around?
Take care.
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