There should be a way to put in upcoming defined contribution so you know what the previous offer amount was when an employee has to make their prior offer election before their current election. Currently, if you change the Defined Contribution amount in the Benefit Subgroup Configuration page, it applies that contribution for both the Upcoming and Current Offer. 

Also, after you open enrollment, if an employee needs to enroll in the Prior Offer, the applicable defined contribution is recognized and applied properly.