Anytime you are looking at migration, there are always many “rammifications”.
The change you are proposing will have to take a whole server offline until you get 2007 running…and there is a decent possibility that 2007 may not run (or run well) on your 2003 hardware. If you migrate and there is a hardware problem you have just been set back by the time it takes you to 1.) Get the necessary hardware and the proper installation, or 2.) Roll back to what you had (probably requiring a reinstall of 2003).
In addition to thw 2007 install itself (assuming it is successful), you now have to worry about updates to the system, and tweaking it to where it needs to be to perform like you had it previously…another time consuming task. On top of this, migrating mailboxes can take a long time. We have several users with mailboxes of 5GB or greater, and these took a while to migrate on good hardware.
I would, under no circumstances, consider migrating during normal hours.
Hope this helps!