First, Yes, they would be a fiduciary for making the change on re-enrollment - if targeting those specifically (and not doing so for the entire plan participant base). Second, there may be *reasons* to be in multiple TDFs (and yes, I'm giving the participants probably more credit than usual). Some may be balancing their in plan portfolio with outside assets. Some may be choosing to be more aggressive with a "portion" of their portfolio without having the burden of managing it through the other fund options. Some may have other reasons. I constantly use as an example of the "misuse" of TDFs by asking whether a client/prospect thinks two 45 year olds should be in the same TDF when one contributes 1% to the plan, and the other 15%?
I learned long ago not to second guess participants (for a variety of reasons, including the fiduciary reason), and to resolve such matters with "education."