The Swancon website currently says: Members of the previous natcon Convergence 2 as well as members of Swancon 33 “The State of the Art” will be eligible to vote in the final ballot.
I believe the intent is to remove the home advantage bias by opening up voting eligibility to the previous Natcon members. That is, you need to be a member of at least one of the Cons - this one and the last one - to be eligible to vote. I need a venn diagram. Basically, it's to include as many people as possible to vote not exclude. So if you're coming to Swancon, you can vote (up to March 21) or if you went to last year's Natcon, you can vote (up to March 21).
Also... some trivia for the weekend, since I was already at the website:
Who is this Ditmar person?
The Ditmars are named after Martin James Ditmar (Dick) Jenssen, a founding member of the Melbourne Science Fiction Club. Martin financially supported the awards until about 1975 and is still active in fandom.