Traditionally, strings that are defined with no length parameter would default to having a length of 255 characters. With the advent of 32 bit Delphi, strings with no defined length default to a long string. Long strings are pointers to a reference counted AnsiString. Long string types are not recommended for use in structures that are intended to be written or read from a disk file. To port your code correctly, you need to explicitly set the expected length of the string in the record, otherwise, a long string is used in place of the original string in the record, leading to unexpected results. Example:

  ShortStringRec : record
    s : string[255]; {this uses a short string - ok}
  end;
  LongStringRec : record
    s : string; {this uses a long string - not ok}
  end;