I don't really have to turn off my board to alter tables do I?

[Gutspiller]

I'm working on installing a hack and it says to turn off my board while I run this:

1) Shut down your vBulletin then alter table - 'user' with this command:

ALTER TABLE user ADD pmunread SMALLINT(6) DEFAULT "0" NOT NULL;

I don't really need to turn it off do I? I've never turned off my board before to install a hack. Can somebody let me know if I need to and what may/could happen if I don't?

Thanks.