The agenda for your event can be found here:
http://www.usfirst.org/uploadedfiles..._OR_Agenda.pdf
I thought I read that you will need to pass a preliminary safety inspection before practice matches, but I can't find it now. At the very least you will be required to have bumpers on your robot per Team Update 11 and if your robot is deemed unsafe you will not be allowed to participate until it is.
You do not need to complete the full inspection to compete in practice matches.
Depending on the magnitude of your changes it may be a good idea to start the inspection process before/while making the changes and it may not. If you make changes after your are inspected you will need to have your robot re-weighed and point out the changes to an inspector so they can verify that no rules were violated by the changes.
If you have any questions at the event about the inspection process I recommend you talk to your Lead Inspector, if you can't find them, most veteran teams should be able to assist you as well.