It is Nippon 2007's policy to not allow you to retain your membership voting rights when you transfer your membership to someone else. This means you cannot sell your membership to someone else and retain the right to vote on site selection or to nominate on next year's Hugo Awards. This is not an uncommon policy; ConJose had the same policy in 2002, as have other Worldcons. "Membership Severability" (the right to separately transfer your attendance rights and your membership voting rights) is not addressed in the WSFS Constitution. Therefore, every Worldcon can decide whether or not they want to allow memberships to be severable or not, and you have no appeal from their decision.
If you want to vote on this year's site selection election and still transfer away your membership, I suggest you go ahead and mail your site selection ballot to the official address now. Yes, I know the official deadline has passed, but if you vote this week, the administrator will (99% certainty) receive your ballot and be able to process it and take it to Japan. Remember, administrators are not really trying to find excuses to disqualify people. The Hugo Awards have a "hard" deadline because they need to count the ballots this week and start preparing the award material (envelopes and trophy plaques). Site Selection voting, however, doesn't actually end until September 1. I suggest you include a note with your ballot telling the administrator that you're transferring your membership after voting it.
After mailing your ballot, proceed with the transfer. Be certain that the person to whom you're transferring the membership knows that the membership is "used" and that s/he won't be allowed to vote in site selection. (S/he will be allowed to nominate in next year's Hugo Awards, and you won't be, unless you buy a membership in Denvention 3 or a supporting membership in Nippon 2007.)
Aside from the matter of next year's Hugo nominating rights, doing things the way I describe above has the same effect as "reserving voting rights" and still abides by Nippon 2007's policy that says you can use your voting rights before transferring your membership but you may not reserve them when you transfer your membership to someone else.