The answer is essentially “yes”. At the population level, there is a tendency for slight decrease in breast density each year, and this tends to be more abrupt in the few years around menopause. One study  showed that only 7% of women who were considered not dense one year were classified as “dense” the following year; similarly 6% of women considered “dense” one year were classified as not dense the following year. For 87% of women, there was no change from one year to the next. Any difference that might affect the decision for supplemental screening would be between women considered to have heterogeneously dense or scattered fibroglandular density one year or the other, and radiologists may differ in this assessment even when there is no true change in the breasts. In a patient with breast density near the threshold, there are likely to be areas in the breast where cancer could be masked: it is not unreasonable to have had supplemental screening even if one’s breasts turn out to be slightly less dense this year.
1. Cohen SL, Margolies LR, Schwager SJ, et al. Early discussion of breast density and supplemental breast cancer screening: Is it possible? Breast J 2014; 20:229-234