latex paint comes off like saran wrap off the roll. Oil paint comes off in chips like potato chips.
Latex will go on latex and oil ok, but oil will not go on latex. Think of latex as a film and oil as having roots to hold on with.
Now, the paint store guy/gal was sort of correct. you should and need to rough up the surface for better adhersion. But, I would add that you should prime the surface after this step. Where the paint chipped you need to apply some spackling to fill in the chip area or the area will show the void area.
Liquid stripper will take too much off, more than you wanted to remove.