Hey there. 
Good question. If your service has been shut off due to non payment, then yes, your upgrade eligibility date can be pushed back, this applies to every carrier. AT&T is obligated to give you monthly services but not an assured upgrade date, which once again does apply to every carrier.
I'm not sure how many times it is done, but as a general rule of thumb, I would try to keep paying the bill on time. If you prove that you have been consistently paying on time, they might let you upgrade as a courtesy.
Hope that helps!
Good question. If your service has been shut off due to non payment, then yes, your upgrade eligibility date can be pushed back, this applies to every carrier. AT&T is obligated to give you monthly services but not an assured upgrade date, which once again does apply to every carrier.
I'm not sure how many times it is done, but as a general rule of thumb, I would try to keep paying the bill on time. If you prove that you have been consistently paying on time, they might let you upgrade as a courtesy.
Hope that helps!
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