Replacing PVC Liner (questions)

I've had my PVC pond liner for about 7 years (dug it myself :-) it's
about 300 gallons and it suddenly sprang a leak. I bought some patch
material and it didn't work (I assume it has more than one leak).
When I was patching I noticed that it looked like it was about to
crack in a few more areas too (near the shelves). I went ahead and
bought new liner and I have a few questions about replacing it!

What are the pros and cons of leaving the old PVC liner in and just
draping the new stuff over it? This is what the guy at the pond store
suggested to me and that just seems like it would be a big mess to me
but I have no idea. I'm inclined to just start over (and I'll have a
few feet of extra liner so I can make it bigger...did I say that? No,
nobody wants a bigger pond!).

Do I need to concrete the edges if I take the PVC out? How stable
will it be? A friend seems to think without the PVC the whole thing
is going to fall apart and cave into the center of the earth.

If I do remove the liner, should I go shelves or no shelves? I've read
"how to" faqs. They seem pretty 50/50. What can you guys tell me from
experience? My pond guy shelves are needed if you want plants. I only
have plants so my fish can have a space to hide, so the added "volume"
sounds good to me.


Thanks for reading my post and extra thinks if you can answer some of
my questions. I plan to drain and move all the landscaping material
tomorrow. My poor fish (just three little koi...they probably need a
little bigger pond anyway) are sick of swimming in a foot of water and
I'm afraid for their lives!