How to know the personalization/ customizations Published from Sandboxes?

We have sandboxes published in the instance since beginning. We would like to know what changes/ customizations were made through each time a sandbox was published.
This is very important due to a new instance is being procured. The changes that were made through published sandboxes are required to know, so that these customizations can be migrated to the New instance.

Please let us know at the earliest.