Yes, i forgot that unfortunately. You shoud switch off the swap before creation. I even deleted the file...

About the creation first - i'm doing that to get automatic swap mount at boot. There are other ways but this one looks easier

@MikeL: I'm not sure 'cause i didn't try, but it sounds reasonable that the swap file gets remounted if you switch cards the same way as it is on reboot - i mean with its full size.