It doesn’t work that way for most people. ( i’m not saying it didn’t happen to you, I’m just saying that there are many people who are able to use a new Samsung account with the classic app without losing any of their information.)
What does commonly happen, however, is that when the new Samsung account is created, a new, empty “location” is created on your account, and that is also set to be the default.
So many people have reported that after being upgraded to the new Samsung account their hub and all devices “disappeared.” But it turned out they really were still there, they were just on a different location.
This issue and the solution is discussed in the community FAQ on the new accounts:
It sounds like what may have happened is that you got a support person who had no idea what they were doing and gave you the wrong advice. Unfortunately, that has been happening more often since last November when apparently they shifted more of the support work to the general Samsung group. Which is, indeed, enraging and awful. It’s just that the issue is not the new app deleting anything. It’s actually adding new stuff, but then also changing your defaults.
Of course it’s also possible that in your case it was a glitch that actually did delete stuff. You never know. 