Changing a value list definition will have not change the data already entered into a field formatted with that value list, though it could change how it is displayed in some cases.
But moving from a "use values from field" value list to a custom values value list is generally not a good design change to make.
Perhaps instead of deleting "customer c" from your table, you should just set a value in a field of that record to mark them as "inactive".
Thanks, just to clarify though I don't want to remove any of the data already there, I just don't want the pick list values to come from the table itself because the problem is that if I want to add say Customer D, I have to temporarily change the field to allow entry of other values, then change a new record to say Customer D so that that value appears as an option for all others. Issue is if you delete that record, there's no more Customer D option, so that's why I want to use the use custom values option so they are permanently in filemaker for that field.
Sorry, but in my opinion, the costs don't justify the benefits for doing that. You loose far more than you gain by making that change--though I have pointed out that making that change won't modify existing data in existing fields if you go ahead and make that change anyway.
The problem is that if I want to add say Customer D, I have to temporarily change the field to allow entry of other values, then change a new record to say Customer D so that that value appears as an option for all others.
Sorry, but this is not the case for a "use values from field" value list. You simply need to add a "new customer" button that creates the new customer record for you--which then automatically adds the customer to the value list. This can even be a special "action value" in the field based value list.
Removing a customer record--except to correct errors such as recording the same customer twice is generally not good business practice as you normally need to retain info on all past customers for legal and tax purposes. But you can mark customers as "active" and "inactive" to control which values appear in a conditional value list (which will also be a "use values from field" value list) so that your list only shows active customers. And yet a relationship that matches by customer ID can continue to show a customer name and other data from your table of customers.
For more on custom value lists and other enhanced value selection methods such as "action values", You might check out the Adventures in FileMaking series. Adventures 1 and 2. They are free to download.
Adventures in FileMaking #1 - Conditional Value Lists (includes details on how to set up a basic field based value list)
Adventures in FileMaking #2 - Enhanced Value Selection (what to do when a simple value list won't cut it.)
Sorry I don't think you understand what I'm trying to do, I've made the change and it works fine but to clarify. We use Salesforce and moved the data to FileMaker. To keep it simple just look at 2 fields in the Accounts table "Account Name" and "Customer Type". In Salesforce the Customer Type was a drop down, with values like "Small Business" "Mid Market" and "Enterprise". So sample data might be:
Customer Name / Customer Type
ABC Plumbing / Small Business
John's Hardware / Small Business
Cupcake World / Mid Market
Network Services / Enterprise
When we moved to FileMaker I imported all the data. For Customer Type I made it a pop up menu as it should be a drop down letting uses create records and pick the customer type from a drop down, or change future ones. Originally for "Values from" I said use the values from the accounts table, and the "Customer Type" field, which of course works.
The problem is that like I said, if say Network Services was the single only account under Enterprise, and they change to Mid Market, as soon as I change that record to Mid Market, the option for Enterprise goes away for all new records because there's no existing records with that value so it doesn't exist in the pop up menu. Same with if I need to add a new customer type, such as "Large Enterprise", it would be incredibly difficult because the pop up menu does not allow users to just make whatever values they want.
So instead the values for that list should be under the "use custom values" or somewhere else, and not pulling them from existing records. I changed it already and everything works fine though. The previous way was super messy, as deleting an account that is the single one and only account with that particular customer type value would cause that option to be removed from the drop down.