First, what we have in a shorthand manner called "lesser change" really isn't "lesser" so much as "not greater" or "not increasing the scope of change," but neither of these rolls off the tongue. Unfortunately, "lesser change" as a shorthand term can mislead people into thinking it means something that it does not. Let's start by looking at what the actual rule for amending the WSFS Constitution is. (Emphasis mine.)
Section 6.6: Amendment. The WSFS Constitution may be amended by a motion passed by a simple majority at any Business Meeting but only to the extent that such motion is ratified by a simple majority at the Business Meeting of the subsequent Worldcon.
So when we say "lesser change," we're actually referring to the "but only to the extent that such motion is ratified" portion of the rule. What this means is that it's possible to ratify a constitutional amendment in part, or to reduce the scope of change of the proposal.
Here's a relatively simple example of what "scope of change" means. Let's pretend for the sake of argument that the existing WSFS Constitution has a provision that says, "No Worldcon may sell a Supporting Membership for more than $50." (There is no such provision; I'm using it as an example because it makes things easy here.) Now assume that an amendment received first passage this year that strikes out "$50" and inserts "$100," thus allowing Worldcons to charge as much as $100 for a Supporting Membership. This proposal then is sent on to Finland for ratification.
At Helsinki, an amendment is made to change $100 to $70. This is within the scope of change of the original proposal, because the existing value is $50 and anything between that number and $100 would be within the scope of change. If the amendment to $70 passes, we could ratify the propsal without requiring an additional year's vote. Conversely, changing $100 to $150 exceeds the scope of the change and would require re-ratification in San José before it could take effect.
So far, so good. But note that simply lowering the cost doesn't guarantee that the change is in scope. Changing the value to $40 would be less than $100, of course, but it would also exceed the scope of the original change. Only values greater than $50 and less than or equal to $100 are within the scope of the change. (An amendment to change the value to $50 would be out of order as dilatory; you get the same effect by simply rejecting the ratification.)
I hope both of these examples are clear. However, some people seem to have the idea that "lesser change" only means "fewer words," which is not the case at all. In the example above, if someone were to move to strike out "not more than $100" and insert "not more than three times the Advance Supporting Membership [Site Selection voting] fee, but not less than $50 nor more than $100," that would still be within the original scope of the change, even though it is obviously many more words.
The reason I bring up the "fewer words" misunderstanding is that I've read some people speculating that someone could move to strike out words from a proposal awaiting ratification in such a way that actually increased the proposal's scope, but because it had fewer words, it would be considered a "lesser change." That's not true at all.
So all of the above examples apply to a relatively simple case where it's relatively easy to determine what the scope of the change is. Where we get into trouble is when we have very complex proposals. In some cases, it is not at all easy to determine if any change at all would reduce the scope of a change. I have in the past ruled that certain proposals could have almost nothing that reduced their scope because they had so many moving parts in them. About the only things that seems to be always within scope is to include "sunset clauses" that require re-ratification at a future date. That's because changing the Constitution for (say) five years is less of a change that changing it permanently.
The arguments about what constitutes a so-called "lesser change" (that is, what sort of changes to a pending Constitutional amendment do not increase the scope of change) have become so difficult that there have been some serious suggestions that we abandon the concept entirely and simply require the ratification to be an up-or-down vote, with no amendments allowed; or that any changes to an amendment pending ratification require an additional year of ratification. As next year's presiding officer, I won't take any position on such proposals, of course, but I will as usual help people construct such proposals and in fact have already done so. Whether such proposals make it to the floor next year is an open question.