The above is a good experience but again it's not about what was on the bike first and what it was changed to but rather what rim you are mounting it on. As you can see at Sheldon's site - http://www.sheldonbrown.com/tire-sizing.html on the chart labeled "Which tire fits safely on which rim?" It is difficult to find a rim where one could safely use both a 28 and a 20 mm tire. In my experience you would be ill-served if you mounted a 20mm on a rim wider than 15mm, especially if you are heavy or on bad roads often, and 15 is even stretching it.