What changes the structural rate of output/unemployment?
Let $L$ now rise. What happens to $u_n$ and $Y_n$?


From the equation, if $L$ rises, everything else equals, then $u_n$ will rise (that is, the fraction will be smaller).

$Y_n$ will also rise and that is a fairly reasonable conclusion - Extra work force will lead to more outputs. 
Now, the misleading conclusion is in regard to the markup. Consider now the labour market. When demand (work force) increases and all else remains equal (supply side - firms offering job opportunities), wages will lower and thus that setup will have a new equilibrium - with lower wages and more work force.

This is my interpretation of your question, although the answer may not be 100% accurate, or an answer that actually answers your question.