"one of the upgrades that is/are being considered is a . . ."
If the situation is that many upgrades are being considered and your sentence is going to continue on to discuss only one of them, then both "is" and "are" would be considered to be grammatical and acceptable as standard English.
The rest of this post is copied from one of my other (lengthy) answer posts, and its info might be helpful to you:
The power of "one": When "one" is the head of a partitive, the presence of "one" can often attract a singular verb, and it can do that even when the semantics call for a plural verb. CGEL page 506 (and especially notice version [22.ii ]):
. . . The relativized element in these examples is object. Where it is the subject that is relativized, the expectation would be that the number of the verb would be determined by the antecedent, giving a plural verb in Type I, and a singular in Type II. In practice, however, singular verbs are often found as alternants of plurals in Type I:
i. He's [one of those people who always want to have the last word]. -- (Type I )
ii. He's [one of those people who always wants to have the last word]. -- (Type I )
iii. He's [one of her colleagues who is always ready to criticize her]. -- (Type II )
Examples [i] and [iii] follow the ordinary rules, but [ii] involves a singular override. It can presumably be attributed to the salience within the whole structure of one and to the influence of the Type II structure (it is in effect a blend between Types I and II ). But it cannot be regarded as a semantically motivated override: semantically the relative clause modifies people. This singular override is most common when the relative clause follows those or those + noun.
NOTE: CGEL is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language.