It's just two days. I have a feeling it's just a ride system test and adjust. If the new sleds were being put on, they would have to be the exact weight of the old ones, otherwise it'd need a longer testing period.
Standard checks? Or maybe they're installing the new bobsleds we're supposed to be getting.
(I'm just guessing. )
The new sleds probably take more than two days to install. They'll probably show up during the next major annual refurb, if Disney doesn't save their appearance for the 50th anniversary of the ride in 2009.