To reverse a conversion:
1. Convert your newly converted item to the original type. However, this is not an undo. This is a convert. When changing back to the original item type, the item will receive new identifiers (item number, etc.). This is because you are not undoing the conversion, instead you are converting.
2. Any fields that do not convert in the new conversion direction will be lost. In order to manually replace those fields, go to the item's Versions tab.
The fields you lost will be noted in the change details and can be manually replaced.
If you had converted the item and then edited it before deciding to revert it back, you will only have the ability to revert as far back as the item of the new type existed (using the version tab). Once edited, it is not possible to revert back to a version where the item was a different type.