Hello,
Apparently for part of the misssion Hubble will have to be completly shut down. Astronauts John Grunsfeld and Richard Linnehan will first have to remove the degraded, old unit, "switching off" Hubble in the process.
It will then be a race against time to fit the new unit and power up the observatory again before the extreme cold of space does any damage to the telescope's instruments.
If the worst happens and Hubble cannot be turned back on, Nasa will have little choice but to abandon the observatory.
This is a partial excerpt from a BBC article, it is also stated that this kind of goes against the general space policy that if something is working well you don't turn it off and hope it comes back on.
I don't want to be pessimestic so here is the good news, if successful the phone booth sized upgrade, Advanced Camera for Surveys will increase Hubbles discovery efficiency by 10 times.
Good Luck.
See ya, VINCE