title: Updating data in lm() calls
body: <p>Is there is an equivalent to update for the data part of an lm call object?</p>
  
  <p>For example, say i have the following model:</p>
  
  <pre><code>dd = data.frame(y=rnorm(100),x1=rnorm(100))
  Model_all &lt;- lm(formula = y ~ x1, data = dd)
  </code></pre>
  
  <p>Is there a way of operating on the lm object to have the equivalent effect of:</p>
  
  <pre><code>Model_1t50 &lt;- lm(formula = y ~ x1, data = dd[1:50,])
  </code></pre>
  
  <p>I am trying to construct some psudo out of sample forecast tests, and it would be very convenient to have a single lm object and to simply roll the data. </p>