Your dates will look off, but you can assign 7 December "1941" as whatever your start year is and go from there.
I can tell you from experience that adding a mass of new material to the data base, while leaving the now unused stuff there (changing arrival to 9999 so it is inactive) can result in problems loading the scenario files, so you will need to do some culling of ships, air-groups, and ground unit 'locations'.
The good news is that because you have some pre-Washington treaty ships in the 1941 scenarios, you already have most of the devices you need to set up all the early 30's ships in the data base (On the allied side, the Omahas, the 4 pipers, the Caroline class descendants, the admiralty class DDs and the like give you most everything you need.)
The first thing you need to decide is if your scenario will be in a non-Washington universe. That will determine if you go with the "treaty cruisers", the 8" armed 10,000 tonners, or off in other directions. In non Washington universe, the USN gets Lexington CCs, instead of CVs, the IJN does not get the Akagi or Kaga as CVs, the RN does not get Courageous and Glorious, and so forth.
"You may find that having is not so nearly pleasing a thing as wanting. It is not logical, but it is often true."
- Cdr Spock