Before you start with the activity, please ensure you have SQL database backup and catalog backup.
1) To backup SQL database and catalogs you have to stop all BE services and SQL (BEWS) Services, copy the folder "DATA" and "CATALOGS" from you BEWS installed folder.
2) Id you want take a screenshot of the updates installed by going in BE console--->Help---->About BE---->Installed updates.
3) Also if you want take a screenshot of the Install Licenses or copy the file Besernum.xml present in C:\Windows.
4) All is you are prepared to migrate the BE server with backup of BE database.
5) For you last question of restoration, just you have ready catalogs or you can catalog the data that is present on B2D or on Tape.
6) Any how you can restore the data fast if catalogs are ready or if not you have to catalog the data which comsumes some time.
7) your backup data is secure till you dont delete B2D or scratch the tape.
Please let me know for further any quereis, so that I can share Best practices to migrate the BE server.
Good luck
Regards
KK