Changing your brand message can be a tricky proposition. There are many reasons that we might think it would be a good idea but is it really. Let's look at a couple reasons why brands are tempted to change their messaging and if they are valid reasons to do so.
Developing a strategic brand message can often take months to fully develop and craft the communication mediums to be effective. During that time we are fully engrossed in the process and idea, and when we come out the other side we are often exhausted and tired of the idea. It is at these times we need to let the brand live. You've put all that work into it, let it gain traction and do the work it was intended to do.
If you've done a good job developing the brand message then you shouldn't need to change it drastically for quite a while and maybe never. This is dependent on your particular industry and your position in it. Your brand should never become stagnant but it should stay consistent enough that your audience recognizes it.
Since branding is about what your customers think about your company, product or service it is guaranteed that your competition is trying to influence the same people you are. Rarely do the actions of your competition require you to drastically change your brand messaging. You should be secure and confident in who you are as a company. If you don't know what makes you great then how will be able to communicate that to the market? This research and exploration should be done before you pursue any design, marketing or advertising.
A strong brand is never reactionary, if you are constantly changing your brand message then you need to do some more research about your market and what role you want to play in that market. That being said, it is important to have matrices to view how your brand is performing in the areas that are important to you. If you are not meeting your goals then perhaps some small adjustments can be made and then collect more data on how the changes are performing over time.