The problem is that there might be cases where you get neither errors or warnings, but rather silently get different results.
Consider how String was an abstract type in v0.3, was deprecated in v0.4, and is a concrete type in v0.5.
Skipping directly from v0.3 to v0.5 would make people miss places where they should have changed String to AbstractString, and then things might break further down the road if for example if they get passed a SubString instead of String.