Problem:

Existing field mappings of base or managed solutions can be deleted but not changed. However, the deletion is then not transferred with the solution. So this is not an option to change a mapping.
Example for this is Originating Lead to Account
In our Example we don't need the Description in the account record.
Actual way to solve this, is to create a new Filed called description and hide the original description.

Suggestion to modify or extend the field mappings:

Conversion:
If the possibility existed to change an existing mapping so that the target field is changeable, the change could be transferred in a solution, and is then also part of the solution layer.

Enhancement:
If the conversion does not work, an extension could be used to remedy the situation.
The idea is to give the field mapping a property with which it can be decided to apply the mapping. With the change this could change then in a solution as administered level the basis and can be uninstalled thereby again.