Ruby has the tap method (1.8.7, 1.9.3 and 2.1.0) that's very useful for stuff like this.
original_hash = { :a => 'a', :b => 'b' }
original_hash.clone.tap{ |h| h.each{ |k,v| h[k] = v.upcase } }
# => {:a=>"A", :b=>"B"}
original_hash # => {:a=>"a", :b=>"b"}