Bootstrap 3 was released in August as a rewrite rather than an upgrade. The two grids from 2.3 — .row with .span* for fixed layouts and .row-fluid for percentage ones — are replaced by a single mobile-first grid, so every template that uses the grid has to be edited.
<!-- 2.3 -->
<div class="row-fluid">
<div class="span8">…</div>
<div class="span4">…</div>
</div>
<!-- 3.0 -->
<div class="row">
<div class="col-md-8">…</div>
<div class="col-md-4">…</div>
</div>The rename is mechanical; the behaviour change underneath it is not. .span8 was eight twelfths at every viewport width, whereas .col-md-8 is eight twelfths from 992px up and full width below it — so an admin table that was never meant to stack now stacks on a laptop with a narrow window. Adding .col-xs-8 alongside restores the old behaviour where it was wanted. The rest of the migration is a list rather than a rule: .control-group becomes .form-group, .icon-* becomes .glyphicon-* and is now a font rather than a sprite, the default button is white instead of grey, and the responsive stylesheet is no longer a separate file. Budget a pass over every template, not a find-and-replace.