For customizable bundles, if the product is removed from the bundle collection customers who have already selected that product will still receive it. However, if the customer logs into their customer portal they will be prompted to adjust the contents to remove that item and select a replacement.

It is recommended that fallback contents and automatic reset bundles are enabled if you anticipate changing the bundle contents frequently. 

Preset bundles are managed in the bundle editor so customers with preset bundles will renew with the updated products as set by the bundle configuration.