The MTA is beginning to straighten out the confusing maze of ramps at the Fulton and Nassau subway stations downtown. That means over the next two weekends some riders will have to exit and re-enter the complex to make the connections they want. They can get free transfers to do so.
The detours are necessary because contractors will be removing the ramp on the western side of the A-C platform, and replacing it with a temporary staircase. Eventually, workers will erect a level concourse in place of the ramps. Permanent staircases will bring riders down to the A-C platforms. The concourse will allow straphangers to get from the west end of the complex to the eastern end without having to go down to the A-C platform to do so.
There's more detail on the MTA's website.