First noticed last week when printing last year's flight activity off of the Mileage Plus summary that the actual fare bucket of the purchased ticket printed out (i.e., W, S, T, L.) It was overridden, as in past coach fares, by F or C when a segment had been upgraded. Today, the 1K web-site mentioned this as an enhancement:
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"To assist in tracking your flight activity—and promotion eligibility—we will be listing the actual booking class for each flight listed on your mileage summary.

Now you will know which class you purchased for every flight listed on your Mileage Plus® Summary. What's more, you will be able to track your progress for promotions that require you to purchase certain fare classes in order to qualify.

Your improved mileage summary will be included in the next issue of Mileage Plus Update print newsletter and My Mileage Plus e-newsletter."