As DJ says, it won't overwrite your exisiting catalog (why should it?...there are some Wedding photographers who start a new LR catalog for each wedding, I've yet to see anyone complaining about LR overwriting an existing one.) and as ssprengel says, it can't overwrite the old one whilst reading from it.
If you're still unsure then just make sure you've got backups (which is always essential let alone when you're attempting something you're not sure of).
Do take care though. My own expereince is that we humans are more likely to overwrite an existing file than the software is.......