I am thinking of giving my party's fighter a +2 greatsword with the ability to cast compelled duel 2 or 3 times per day (save DC 13). Does anyone think this will be enough to change it's rarity?

i'd say usually no if its the only 'minor' feature, I imagine it as it takes two 'minor' features to equal one more standard feature, except the step up from mundane to common to uncommon (most common magic items are really only one minor feature).  BUT I'd say there are some items out there though that already have major and minor features, so in those cases, yes, it'd bump up the rarity. 

all a judgment call and all imo of course

for what its worth, here's a source that takes that approach  https://www.dmsguild.com/product/321143/Armor-and-Weapons-Foehammers-Book-of-Secrets

Thanks!