I saw an older post on transfering ECG data, which pointed to calling a support line (1 855 195 0509, extension 6). That post was marked 'solved' so I was not able to post there, but I did want to share what I learned with other users.
I called the support line and was told that ECG data cannot be transferred from one device to another. I was told this was a data format compatibility problem. I expressed that it didn't make sense that data from a Samsung Note 9 would be incompatible with Note 10+ when using Samsung Smart Switch app to transfer, and still using the same Samsung Watch 3.
Then I was told a different reason, that the data could not be synced because of "FDA regulations".
I was told our only option is to export the data from the old device manually, BEFORE it is reset. In other words, ECG data is not backed up to the cloud along with other Samsung Health data, as both Health and Health Monitor apps imply with their 'sync' options.
There is no indication of this requirement for manual backup using Samsung SmartSwitch, although SmartSwitch does give warnings about other data that must be transferred manually, like WhatsApp, etc.
I did export my data manually, which is in PDF and CSV formats. There is no good reason that this data cannot be imported into HealthMonitor app, other than that the app does not support it.
I don't appreciate being lied to by Samsung support.
This is a significant gap in both the Health, Health Monitor and Smart Switch apps that needs to be filled.