You can both buy a new AP and renew an existing one (within 30 days either way of its expiration) by calling MS. You can also do it online at the AP site...but while I know I did do that once, I remember it being somewhat confusing and complicated...
Yes, you get both the
DVC member discount and the renewal discount (I'm on my 7th consecutive year of renewal).
If you want to buy a new AP, you can buy one "now" and just hold on to the voucher til you're ready to use it...you exchange your voucher for the AP itself at any Guest Services and it activates the first time you go into a park...so, you could trade your voucher for an AP on one trip, but keep it unused (for years & years!) and it won't "start" til your first entrance into a park.
For a renewal, though, you keep your same expiration date no matter what.
So, renewals (even w/the discount, which as I recall is around $35-$40) may or may not be worth it, depending on the timing of your next trips...we've been going 3-4 times/year for 5-6 years now, so I don't even think about not renewing...next year I'll have to start thinking about that, though, as my DS will out of school and into the work world of vacations and less time flexibility, so our WDW patterns will change...
Anyway, renewing (or buying) through MS is very easy...
For people who are more computer-literate than I am, renewing (or buying) online may also be easy--but I remember thinking it was SO much trouble to figure out I just started planning on calling to do my renewals...