If you have a thru-hole board that is truly performing well, and "change it from classic DIP to surface mount" is an accurate description of the task, it shouldn't be too difficult. SMT components are usually smaller than their THD counterparts, so it's almost always safe to substitute SMD for THD, make minor adjustments to trace routings, and accept a layout with a lot of elbow room around all the components. The problems start when somebody says, "And while you're at it, find a substitute for this obsolete part.", or "And make the board half its size.", or "And add this new feature to the board.".
Do you ever get out to the Chesterfield area? Send a private message to me.