SBS 2003 with SBS 2008 (during a migration timeframe)

Another question for the SBS specialists here :

one of my customers needs a slow migration from sbs 2003 to sbs 2008 (it is
almost impossible to do this in an overnight or overweekend way)
so my questions are
- when we start setting up the sbs2008, while 2003 is still running, should
we expect issues? (will they "fight" with each other?)
- at the moment, we move date (a file share) from 2003 to 2008, would users,
who are still logging on to the 2003 DC, access a share on the 2008 SBS
primary server?

this situation would be for about 10 days

thanks

L.