How to change textfield to textarea

On my update page created by DataAssist, I need to change a textfield to a textarea. How can I do this in a page that is already created? I tried changing the code from:

<div class="lineGroup"> <label for="describeYourself" class="sublabel" > Describe yourself:</label>
<input id="describeYourself" name="describeYourself" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("usersupdate","describeYourself"):"".$row_WADAusers["describeYourself"]."")); ?>" class="formTextfield_Medium" tabindex="29" title="Please enter a value.">
</div>
to <div class="lineGroup"> <label for="describeYourself" class="sublabel" > Describe yourself:</label>
<input id="describeYourself" name="describeYourself" type="textarea" cols="50" rows="12" maxlength="1000" value="<?php echo((isset($_GET["invalid"])?ValidatedField("usersupdate","describeYourself"):"".$row_WADAusers["describeYourself"]."")); ?>" class="formTextfield_Medium" tabindex="29" title="Please enter a value.">
</div>

but that didn't do it. I'm guessing the class in that last line has to be changed, but I don't know what to change it to.

Thanks!