Adding tapes
1. You'd add tapes when they're new and wanted to have new scratch tapes. Best bet is to use bar-codes with this, as it makes inventorying and identification easier.
2. If you're running a rotation with your tapes, your tapes will be changed accordingly. I'd suggest keeping your tapes separate somewhere safe. This means you won't get them mixed up.
3. You'd also add tapes into a rotation if tapes were faulty/you needed more.
Changing tapes
1. Faulty tapes - as above.
2. To bring in the new week's tapes - ie. swop out Week 1's tapes with Week 2's tapes.
3. For restore purposes - you needed a specific tape.
Like Ben L. said, more information would be needed. This all assumes you're working well with the HP Autoloader 1/8 G2. If not, you can pick up some technical docs here:
http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&lang=en&cc=us&prodSeriesId=3319912&prodTypeId=12169