As migration completed successfully, I have the answer to my question.
Once SM is broken-off, the 'volume status -v' command correctly shows 64-bit now, instead of 32-bit as seen before the Mirror was broken-off. All the 32-bit snaps remains as is, they will be cycled-off/deleted before the data is moved to cDOT.
Internally, WAFL is runing a background task to upgrade the indirect blocks to 64-bit which can be seen via - wafl scan status command from - advanced mode.
Are blocks in Snapshot copies also converted to the 64-bit format?
Answer: No. Snapshot copies are read-only and are not updated by the expansion process. The expansion process updates only the indirect blocks in the active file system.