Not that the listed prices do not include the 6.5% Florida tax.
Currently, a one day pass (adult) is $79 plus tax.
If you use your 1 day pass as a credit towards the price of the DVC AP, you'll need to give Disney about $330, including the tax (assuming my math is correct).
We just happened to do this exact thing when we returned to WDW earlier this month. My wife and I each had a 1-day Magic Your Way left over from June 2009 that we applied toward annual passes (AP) without the water park option. With the DVC discount (which is $100/pp), the total cost with tax was $660.30 for two adult APs. Keep in mind that the amount of credit you get from your unused tickets might vary from what we received. Disney uses a straightforward but still complicated formula in determining the value of unused tickets.
You can obtain your APs at any WDW Guest Services. We did it at one of the GS locations in Downtown Disney. Each of the Parks has a GS just outside the Park before you go through the turnstyles.
One word of caution: When you get your APs, check the expiration date on the back of the AP. When we converted our unused 1-day tickets to APs, the Disney computer system somehow used the original issue date for the 1-day ticket (6/11/2009) as the "issue" date of the new APs. Thus, the APs showed an expiration date of 6/11/2010! We noticed it the next day and went to GS. The CM at GS easily corrected the problem and issued us new APs with the correct expiration date of 2/28/2011, 12 months after we bought the APs.