When you create a world with the same seed it will be different in 1.13 from 1.12... but if you create a world in 1.12 and generate chunks in 1.13 the 1.13 generated chunks would be the same as the 1.12 generated chunks. Spawn space in the 2 versions with the same seed:
1st image: 1.12
2nd image: 1.13