Every good AdWords account manager has been here before. You have an account that you've been managing for a while now and you were finally able to switch it over to CPA or ROAS bidding. During a regular checkup you spot some hot new queries that you want to target. A deep feeling of anxiety fills the pit of your stomach because you have FOMC, that is Fear Of Making Changes to your AdWords account. The problem is, you don't want to throw off any of the success the campaign has already had. You also do not want to clutter the account by creating new campaigns that will overwhelm you and/or confuse the client. So what should you do? Think like the algorithm.
The AdWords automated bidding strategy algorithm cares about one thing and one thing only. Conversion rate. It analyzes hundreds of variables for each a impression and generates a historical conversion rate for each set of variables. In this case, the only variable that you would be changing is the keyword. So, if your campaign has a historical conversion rate of 5% overall and the ad group you saw the new query in has a conversion rate of 8% but the queery only has a conversion rate of 3% then adding that query as a new keyword will most likely cause the algorithm to overbid and decrease the overall campaigns performance, justifying your FOMC and causing you to enter a frenzy of reverting changes.
So how can you add new targets two existing campaign without blowing it? Make sure that if you are adding the new keyword to an existing a group that it has essentially the same conversion rate. If not, look for an ad group you could add it to that has similar conversion rates. Start the keyword out with manual enhanced CPC bidding based on the historical conversion rate of the original query. Once it runs for a few days and Google clicks and data on it then you can switch over to the campaign bid strategy and the Machine will take over.
Don't let FOMC keep you from adding new keywords to your clients' accounts. Do make sure that any tweaks you make do not introduce massive new changes into the algorithm. Ease in your changes.