Use Underscore's methods (since it's required by Backbone, you have access to it):
new modelsecond(_.pick(_.clone(modelfirst.attributes), 'fld1', 'fld2', ...))
To select all the lower case attributes, you should be able to do
new modelsecond(_.pick(
_.clone(modelfirst.attributes),
_.select(_.keys(modelfirst.attributes),
function(attr){ return /^[a-z]+$/.test(attr) }))