In theory this is a funny idea but I think it would be a huge waste of time. Ruby syntax has changed and added so much since since 1.8 that you'd be stuck fixing thousands of syntax errors.
Ruby has had a lot of syntax changes over the years that are incompatible with an older interpreter. An easy example is the 1.9 shorthand hash syntax for symbol keys `{ name: "pants" }` instead of the hashrocket syntax `{ :name => "pants" }`. More recent examples would be keyword args or pattern matching.