timing of disclosure

When there is a reduction in fees (paid from plan assets), what is the required timing to inform participants/sponsor?

My understanding is that the timing is the same (30-90 days in advance) regardless of whether there is an increase or a decrease.  However, I understand the DOL recognizes that there are some cases in which that much advance notice is not possible, and in that case notice can be provided as soon as reasonably practicable.  The example the DOL uses in that case is one in which an investment is eliminated because it is no longer prudent.  This is not exactly on point, but FWIW we have sent change notices after the fact when a vendor has retroactively reduced quarterly fees.  We felt that the fiduciary's responsibility for keeping participant fees/expenses as low as possible outweighed the advance disclosure requirement.  (I hope I am right!)