The parts that aren't obviously for the totally different 16 are all actually compatible with the few exceptions indicated on each special part.
Like the AMD motherboard will need a new wifi card and ram I think, and this is indicated on the page for that motherboard. But it fits in the existing cases. Same for the new speakers, battery, hinges, screens.
There are some special chromebook versions of some parts and even they fit physically but I think just aren't supported in the bios except with the chromebook mother board.
So far until the 16 there have been many small updates but all compatible and no breaking changes.
The 16 is one new different platform. I think one new platform after 3 years is not a "constant iterating new chassis..." or however that was worded (phone won't show me that post until after I hit reply)
Basically it's not a problem.
The new 13th generation Intel processors, with a mainboard that should fit in your Laptop 13 (11th gen) chassis, should give better battery life, and I do think the chassis is about the same, though I think its got the stiffer back and hinges than your 11th gen (though you can order these replacement parts too!). I believe the new Ryzen version (coming out soon, I've pre-ordered) should also be quite good, but we don't have the benchmarks for it yet.