Drop-in resolution
ETA 2836-2 vs ETA 2804-2: do they swap?
Whether the ETA 2836-2 and ETA 2804-2 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The ETA 2836-2 (25.60 mm x 5.05 mm) and ETA 2804-2 (25.60 mm x 3.35 mm) are near in size but differ in height, dial feet or layout, so moving between them means a new dial, hands and often a case adjustment.
The ETA 2836-2 is the day-date sibling of the 2824-2, in dressier and tool watches that show the weekday. The ETA 2804-2 is the hand-wound date member of the 2800 family, the manual sibling of the 2824-2.
They differ in height (5.05 mm vs 3.35 mm), calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3), winding behaviour. Same 25.60 mm diameter as the 2824-2 but taller at 5.05 mm because of the added day disc. A 2824-2 will not fill a 2836 dial: the day window sits empty.
Matching diameter and height is necessary but not sufficient. A donor is only a true drop-in when the dial-foot positions, hand-hole sizes, stem length and calendar layout also match. Two movements can share a case and still need a new dial, re-broached hands or a cut-to-length stem.