As always in such things, there are fun metaphysics questions to ask. I first built an ordinary Lack table out of one of the two kits I bought. I used it for a couple of days to figure out the height I wanted for the final table (I tried putting a Settlers of Catan box on it, and the laptop on that--too high--and I tried putting a second Lack tabletop on it--just right). Then I built the new table. Now imagine that I proceeded as follows (I didn't--I used the legs from the other kit instead, as it happened). After building the original table, I cut segments off the bottoms of the legs, thereby shortening the legs. That wouldn't destroy my table, surely. I would just have a table with shorter legs. I then attach the segments to the second tabletop, and then glue that on top of the first. Would I still have the same table as I started with, albeit no longer a Lack table, even though the tabletop of the first table had become a shelf? What if instead the tabletop of the first table become the top of the new table? That would require more disassembly, though.
Also, although that's not how I did it, I could have started with two pre-made Lack tables, shortened the legs of one, and glued it on top of the other. Would the two tables have been destroyed, replaced by table parts? Would the two tables have continued to exist, but now being mere table parts instead of tables? Or would the two tables have continued to exist, and to be tables, but they would now be a part of a new, third table, giving yet another counterexample (besides, say, Richard Gale's doggy door) to the principle that no object of a kind K can be a proper part of an object of kind K? Would it make a difference if they were joined by Velcro instead of glue? It would be odd if the exact choice of fasteners mattered.
Would it matter whether I used them separately? Or hoped to do so but never did?
There are also fun metaphysics questions to ask about the parts which came in the Ikea box. It is very natural to say that those parts in the box are the parts of a Lack table. But what if they are never assembled? Does that mean that the Lack table exists in the box, albeit in disassembled form? What if I build two ordinary Lack tables, but mix up the parts and screw on the legs from each set into the tabletop from the other set? Are there now four tables, two in disassembled form (that they are screwed into each other--Lack tables normally only use four double-sided screws to put together, and the legs can be easily untwisted from the top) and two new ones assembled? That doesn't sound right. So maybe the kit contains the parts for a Lack table, but the parts of a Lack table only if a Lack table is later made precisely of them.
All these questions are fun, but in Sider's sense they are non-substantive. But questions about existence and identity are substantive. I conclude that I made no tables. (Obviously, I've switched from carpentry language to metaphysics language somewhere in the post.) I just rearranged some fields or particles.