The first example are keywords that have a special purpose in C++: the following is legal in C, but not C++.
int class = 5
These errors are easy to fix: just rename the variable.
In C, pointers can be cast to a void*, which needs an explicit cast in C++. The following is illegal in C++, but legal in C:
void* ptr;
int* intptr = ptr;
Adding an explicit cast makes this work, but can cause further issues.
In C++, you may not skip initializations with goto or switch. The following is valid in C, but not C++:
goto foo;
int skipped = 1;
foo;
These bugs may require redesign.
