Drop-in resolution
Miyota 8217 vs Seiko 6R15: do they swap?
Whether the Miyota 8217 and Seiko 6R15 share a case, and what changes if you move between them.
Different footprints. The Miyota 8217 is 26.00 mm x 5.67 mm and the Seiko 6R15 is 27.40 mm x 5.25 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Miyota 8217 is the 8200-family member adding a 24-hour indicator alongside the date. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.25 mm), calendar layout (centre seconds, date at 3, 24h sub-hand vs centre seconds, date at 3), winding behaviour. An 8200-family caliber at 26.00 mm x 5.67 mm that adds a 24-hour sub-hand to the 8215's date layout. It shares the family footprint but needs a dial with the 24h sub-register.
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.