You cannot add, delete or reorder the local variables and parameters in a function currently active on the stack. You cannot change the type of a local variable, file static, global variable, parameter of a function. You cannot add any function calls that increase the size of the parameter area. You cannot change a local or file static or global variable to be a register variable, and vice-versa. New structure fields can be added at the end of a structure object, not in the middle of a structure.
New fields are only accessible by the modified files.
Old structure fields remain intact, no swapping of them are permitted. If the redefined function is in the call stack but not on the top of the call stack, the modified code will not be executed when the execution resumes. The modified function will be executed when it is called next time, or a re-run. Breakpoints in the original source file are moved to the modified file. Breakpoints in the already instantiated functions on the call stack in the original file are lost.