I am following an e-book. E-book is explaining about using Replace Entity. A cylinder is extruded up to the inclined flat surface of the main structure. After that flat surface is converted into arc like surface, according to e-book final result will end up with an error message and Replace Entity will be used to repair it. 

In my sketch error message is not coming. Extruded cylinder surface is also converted into curved surface.

What is the reason?