Can colonists auto change clothes when worn below 50% instead of complaining?

There is a much better way of doing this: managing uniforms. This can be done from the Restrict menu at the bottom (Alpha 16; not sure if applicable for 15).

You start out with a few preset uniform options: Anything (everyone is set to this by default), Soldier, Nudist.
The cool thing is that you can edit the existing options.

If you load the "Anything" option, you can change what is allowed and what is not allowed. If you set the quality slider to only allow clothes that are between 51% and 100%, your colonists (that are set to Anything) will never wear worn out clothes that carry a negative mood debuff. When a piece of their gear reaches 50% durability, they will take it off.

Caveat: When you do this, your colonists will never wear worn out apparel. Even if it is the only option they have. This may be important if you need clothes for warmth (e.g. boreal forest or ice sheet); because in those cases it's better to wear worn out clothes than it is to avoid the mood debuff from wearing worn out clothes.

I tried this today and can confirm that it works. If there are available clothes of better quality in a stockpile, the person will find them and change his clothes.