SVN reverse merge?

My SVN repository was perfect at revision 497. I performed several bad commits, so that now it is at revision HEAD. I see that to rollback you should use a command like this:

svn merge -r HEAD:497 .

while in the working directory (and the working directory is at the HEAD revision). But is that the right command? Or do I use HEAD:498? or 496? I already ran that command and the current revision doesn't appear to be the same as 497, because when I update -r 497 (or when I have a 497 working copy and I update -r HEAD), a lot of file updates occur.

Do I have some sort of fencepost problem, where HEAD:497 actually reverts to 496, or something? Or is it that when I update, SVN goes back through each revision, despite the fact that the HEAD and 497 are identical?

Edit:

Just to clarify, as I said earlier when I update between 497 and my merged HEAD, I see a lot of file changes take place. I thought that if 497 and HEAD were identical, it would detect that, and no file changes would occur; it would simply say "Updated to revision 497." So is my command wrong, or is this thinking wrong? (and if so, why?)

If your repo was in pristine condition at revision 497, then I think you're correct, you need to do a:

svn merge -r HEAD:497 .

That merge command will only change files in your working copy, so remember to also commit the changes to update HEAD in the repo.

After you do the merge, followed by the commit, try comparing revision 497 to HEAD and they should be identical.

If you want to undo r123, you need to svn merge -r 123:122 .

This means you need to run

svn merge -r HEAD:497 .

To verify run:

svn diff -r 497