Best way to switch the signed-in iTunes Store account on an iPhone without affecting apps
If you switch your iTunes account on the phone from account A to account B all your apps from account A will still exist on your phone and will function properly.

The only issue you will face will be with updates. Every time an update wil exist for one of the apps in account A, you will get the numbered badge on the app store app. However, you will not see the update in the app store itself until you switch accounts back from account B to account A.