Behaviour change: when updating, never drop valid bounds

* Don't add lower bounds when only updating upper bounds. There is no
  need to do that, the user can achieve the same by just updating both bounds.
* If there already is a lower bound/upper bound that is more permissive,
  don't change it. Why should the package no longer build with the old bound?
  If you still want to do that, you can achieve the same by first
  dropping the bounds for that package and then updating again.
* Don't drop upper bounds when only updating lower bounds. The same
  effect can be achieved by using the drop feature.