In order to not mess up your data when changing custom fields, you have to follow the below guide - be aware there are different rules for different types of fields.
Multi select list (you can change the values)
The values in a multi select list is set up by a number in front, for exampe:
01: Rabbit
02: Dog
03: Cat
As long as the number in front of the value doesn't change, you can change the name of the option. Therefore you can for example change Rabbit to Horse, as long as the number 01: stays uncorrected. All entities in the system which had a mark in Rabbit, will now have a mark in Horse.
Are you to add a new value to the multi select list, or even delete an existing value, you first need to create the new value, for example 04: Mouse.
To delete the option 03: Cat, you must first through an Overview do a massmodification, where you either remove or add a new value from the multi select list. When there is no data left with a mark in 03: Cat, you can then delete the value including the number in the multi select list.
List (value CANNOT be changed!)
This is a drop down list where you can choose one option. The options are listed and divided by a *. The first option on the list is always choosen as a standard. A typical List can look like this:
-- Choose --* Option A * Option B * Option C
You can always add more options to the List, and move the options around. However you CAN'T change the text of an option (not even small or capital letters), because if you have existing data with this option, the reference will disappear, and the standard option will be the new option.
Therefore it is NOT possible to overwrite a value and therefore change it.
If you want to change "Option A" to "Option a", the new value is a new option in the system. Therefore you will have to create a new option called "Option a", and move all entities from the original option to the new one, through a massmodification. When there is no longer any entities with the original "Option A", you can delete the option.