Another option is to put the new SSD into an enclosure (like this) and then 1) use disc utilities to format the new SSD to APFS / 2) then install Catalina - 3) then boot from the external drive and use migration assistant to migrate all the apps and settings etc from the internal drive
this should give a perfect copy on the external SSD in the enclosure that you can easily swap with the HDD inside the MBP2012 after you test it
Always good to do a separate back up with CCC (or similar) "back up software" like Taz mentions above
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Actually, you need both - one is system and one is data. SuperDuper or CCC are way to go, they will handle this properly. It is becoming rather complicated.