I have watched some of Getty's images sell at auction for a decent price, but it depend greatly what was in the images. Some didn't bring much money at all if they were obscure or viewed as unimportant. It is possible that some people will stop paying and use the free service, but many will want to access and purchase the prominent photos, which are less likely to be offered for free.
There will undoubtedly be some options that are available with the pay service. However, getty users that do not need those features will switch over to the free version if they realize there is one. There will also be some people that don't pay attention and continue to pay for services they don't need just due to ignorance.
Current Getty customers will most likely continute to pay for the service of using the images. This is because paying customers are likely to be using the images for projects that they would not want to have the Getty image logo on, as the free images will have. The images would be free from logo and advertisement if they paid.
No, I do not think that current Getty customers will stop paying for their product and use the free service, because there will still be photos and features that they will want to pay for. In fact, it will likely result in more business, because more people will use the website and their services.