I just got this offer also and have been flying a bit this year - 43K BIS, 19 segments.
The note that the companion ticket is not upgradable that is usually on companion certs is not in the T&C. But remember
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Offer is subject to change without notice. Other restrictions may apply.</font>
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gfowler-ord-1k
Visit my
web page on how UA is not honoring their terms of the Early Bird promo.