Changing the WSFS Constitution requires the vote of two consecutive years' WSFS Business Meetings. The second year's meeting may change what the first meeting passed, as long as the changes do not increase the scope of the amendment. (Deciding what represents an increase in scope is not always obvious.) The WSFS Constitution includes the definition of the Hugo Award categories and the process for administering them, the mechanism for choosing future Worldcon sites, and a bunch of other, smaller things that I need not go into here.
Because most of the discussion at the Business Meeting tends to be about constitutional changes, some folks think that everything requires a two-year vote. However, there are things the BM can do that aren't constitutional amendments, including passing continuing resolutions, changing the standing rules, or adopting certain resolutions where the constitution explicitly allows a single meeting to suspend a constitutional requirement.
- Continuing Resolutions: If you look at the WSFS Rules page, you'll see an entry for Resolutions and Ruling of Continuing Effect. This document, maintained by the Nitpicking & Flyspecking Committee (yes, that's really it's name), lists rulings made by the Business Meeting Chair (or the meeting itself) likely to have the effect of precedent, and also resolutions having an ongoing effect. For example, the 2013 BM passed a resolution endorsing the the activities of the Worldcon Heritage Organization, a non-profit 501(c)(3) charity set up to preserve the history of Worldcon. (In effect, this gave WHO a license to use the WSFS service marks.) Resolutions are not actually binding upon Worldcon committees because the only way you can bind Worldcons is to modify the Constitution, and even then Worldcons have the nominal ability to ignore changes that impose additional financial demands upon them if the changes were adopted after they were seated. (Currently this means the two Worldcons following the one where the change was ratified may ignore a financial-constraint change if they choose to do so.)
- Standing Rules: The Standing Rules for Governance of the WSFS Business Meeting (also linked from the WSFS rules page) are rules that modify the parliamentary authority and that are primarily concerned with the way the Business Meeting organizes itself. A highly technical reading of Robert's Rules of Order makes it clear to me that the SR is actually a combination of documents that RONR calls "Standing Rules" and "Special Rules of Order;" however, WSFS practice on this is so engrained and traditional that I've given up any thought of trying to separate the two types of document. The Standing Rules can be amended by a single BM by majority vote; however, the change doesn't take effect until the following year unless the BM votes (2/3 required) to adopt it immediately.
- Constitutional Suspension Resolutions: An organization's constitution/bylaws generally cannot be suspended unless the document includes explicit provision for doing so. WSFS has a few such cases. The most common one is a Hugo Award eligibility extension, which requires a 2/3 vote to extend eligibility on the grounds of limited distribution. Much less common (in fact, I don't think it's ever happened) is the suspension of the 125% rule. Worldcons are not allowed to charge more than 125% of the Advance Supporting Membership (Voting) Fee charged in their election for Supporting Memberships. For example, the ASM in the 2017 Worldcon Site Selection is $40. Therefore, the 2017 Worldcon, whoever wins, may not charge more than $50 for Supporting Memberships. (This rule lasts until 90 days pre-con.) However, an intervening WSFS Business Meeting (and there are two of them) may by majority vote authorize a higher value.
I suggested to the 2017 bidders that they may want to jointly propose a resolution to this year's Business Meeting giving themselves a bit more "headroom" to raise their Supporting Membership cost, possibly as high as $60. So far, I've not heard back from anyone, so I guess they're content to stick to not more than $50, which is what MidAmericon II is charging.