How to manage a group well is always a current topic.
- Listen: (as Sam’s list says) and…
- Acknowledge: …to help the complainer(s) know they’ve been heard.
This is often a case of A Complaint is a Gift (Amazon book link) and this probability should always be investigated early on, if not immediately. Sam can probably write a book about the times he has been criticized harshly and it turned into a huge step forward. The viewpoint is not making lemonade out of lemons (dilute and add sweetness). The viewpoint is “thar’s gold in them thar hills.” Find that gold.
Carping complaints come from:
- Not feeling heard (and feeling not heard, especially).
- Feeling misunderstood.
- Feeling powerless.
Users can feel excessively jerked around if changes are made that cause them discomfort or disrupt their routine and they haven’t been consulted or given a chance to digest the value of the change. (How much time have we all invested in learning Discourse in its current implementation?)
SIDE NOTE (click to unfold):
I’ve come to realize that we have a “tyranny of software” at the moment. We regularly depend on software that doesn’t seem to care about how it affects us. This is what makes systems like Discourse stand out: you can tell it was designed and made for users and admins and the group/community. Too many developers take shortcuts and then they (and administrators) shrug and say things like “so it’s two extra mouse clicks, what’s the big deal?” and you want to pull your hair out and shout “I do those two extra mouse clicks 50 times a day and so do thousands of other people! You, on the other hand, saved maybe an hour of coding!!” So unexpected or unwanted changes to a system tend to easily invoke that tyranny that we all deal with regularly–and wish we didn’t have to.
I once read about a study of the dynamics of international diplomacy where one nation holds the upper hand in a significant imbalance of power. For example: The United States and Cuba trying to cooperate. The main dynamic is that the party holding less power and influence easily feels neglected and dismissed. This, understandably, leads to upset from the idea that the other party doesn’t care and just does what it wants, regardless of anyone else’s needs and wants. Sam’s point immediately below and the one above address this feeling of powerlessness. Your members might be screaming “Stop this roller coaster! I want to get off!!” 
Some folks love roller coasters Others HATE(!) them. The safest approach is to drive a bus instead, gently and smoothly. Better yet, drive a train; they speed up and slow down gradually, they don’t make sudden turns, and they carry many more people.
I would add that a system administrator should take time to roll out all changes, and take longer on the more major ones. Have a sensible migration plan and give advanced notice. Use the advanced notice to find out ahead of time if anyone is bracing for a major disruption before they even know whether there will be one. In other words: are they primed to hate it? Phase the migration plan so that you can let the dust settle before plunging over the next waterfall. These settling points–the calm, soothing lake after the waterfall–are a great time to ask for feedback and make sure everyone feels involved and heard.
I also read about another study where a researcher hired an actor to participate in work groups and stir up negativity. All groups failed, as might be expected, except for one. It turned out that this one group had a member whose Father was a diplomat and she/he had learned how to fully acknowledge people and make them feel heard. This prevented any upset from developing, even when it was intentionally being created.
The only safe approach is to address these constructive factors first, while pointing to established (and preferably accepted by all) policy designed to keep the negativity off the lines. After that, it may be necessary to put a head on a pike and display it at the city walls. If you arrive at that point, do it without hesitation, but make sure you’ve arrived step by step, gradually.
While doing the above, you can also remind everyone of the purpose of the community and the long-term goals. That tends to make immediate, transient discomforts less significant. (This is a sideshow, though, to the main actions above. It’s a mitigating action, not the actual solution that should be getting the most attention and energy.) It’s a good practice anyway to always and regularly promote the mission, goals, and purpose(s) of the group or activity. This helps people focus on the right “game”. As a leader, you must give the group a worthwhile game to play and a goal to have and pursue or the game will become getting you.
NOTE: The attacks on new members is curious. Are the new members trying to support the changes and invoking the veterans’ ire by doing so?