Warning:
LEAVE THE ORIGINAL BOARD RUNNING UNTLL YOU HAVE READ THIS AND DONE A FEW TEST IMPORTS
"I've turned my board off and done an import and it didn't work 100% correctly first time, my users are upset, fix it now"
Isn't really the best way of getting a board imported.
Right, on we go.
Make a back up of your source board and do the initial trial run import from that, not the live board.
ImpEx only ever reads from a source board it will never alter source data. Though running an import against source data that is being updated can have some strange outcomes for the import as well as the source board, also it will cause a lot of load on the source boards database and could impair the performance of your source boards community.