4.5 isn't the section used to determine the underlying type of a non-explicitly typed enum, it's 7.2 (paragraph 5 in the old standard and paragraph 6 in the new standard). Both versions of the standard contain the wonderfully non-specific line "It is implementation-defined which integral type is used as the underlying type except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int." Note that it says the value of an enumerator not that range of the all enumerators must simultaneously fit within an int or unsigned int.
Well, 4.5§3 states
A prvalue of an unscoped enumeration type whose underlying type is not fixed (7.2) can be converted to a
prvalue of the first of the following types that can represent all the values of the enumeration (i.e., the values
in the range bmin to bmax as described in 7.2): int, unsigned int, long int, unsigned long int, long
long int, or unsigned long long int. If none of the types in that list can represent all the values of
the enumeration, a prvalue of an unscoped enumeration type can be converted to a prvalue of the extended
integer type with lowest integer conversion rank (4.13) greater than the rank of long long in which all the
values of the enumeration can be represented. If there are two such extended types, the signed one is chosen.
which would seem to apply to enums, no?
7.2 §5 states
Each enumeration defines a type that is different from all other types. Each enumeration also has an
underlying type. The underlying type can be explicitly specified using enum-base; if not explicitly specified,
the underlying type of a scoped enumeration type is int. In these cases, the underlying type is said to be
but, this isn't a scoped enum (enum class / enum struct), so the type is not fixed and 7.2 §6 states
For an enumeration whose underlying type is not fixed, the underlying type is an integral type that can
represent all the enumerator values defined in the enumeration. [...]
Which would seem to lead to the same result as 4.5 §3
In any case, yes, C++11 is not backwards compatible with C++99. In fact even the auto keyword used to have a different meaning.