Can a formula field be converted to read only after a condition is met?

Re: Big Problem  No problem!

Snapshots to the rescue.  It's not really the "formula" you want locked, it is the lookup of the rate that you want frozen.

So I would rename your current lookup fields to be called like [Rate (lookup)] and then per the help text below, make a new field called [Rate] or if you prefer [Rate (frozen)] or perhaps [Rate (snapshot)].  Set the field properties to snapshot (freeze) the value of the lookup field.  Remember to initialize the field with the checkbox when you create the field.

Then look for all uses of the field [Rate (lookup)] and replace with [Rate (frozen)]

Here is help text on Snapshots.

https://help.quickbase.com/user-assistance/setting_up_snapshot_fields.html

Thanks, this is definitely educational and new information for us, but we are working with a formula numeric field within on table and the snapshot feature doesn't exist for this field type unless I'm missing something. Also within our formula numeric field, there aren't any lookup fields that we could utilize snapshot for that would solve our problem.

Is there a different solution for a formula numeric field, where we are plugging in specific numeric values based on other lookup fields?

OK, so plan B.  I guess that you have a hard coded formula field with the calculated rate which depends on the location.

I suggest setting up an Action to be triggered when the record is marked as Paid.  This would copy the calculated amount into a plain numeric field which of course would be frozen.  An Action will be more reliable than a form rule and also will work when multiple records are updated by say Grid Edit.

Now, the issue you will run into that on the surface it will appear that a Action can only Add a record or edit child records and what you want to do is to act on the record itself.

The Technique, which is perfectly fine to use is to set up a Report Link field which will define the set of records to be edited to be the record itself.  So you configure on the left side of the Report Link field properties to be the [Record ID#] field and then navigate on the right side to the app and the same table and the same [Record ID#] field. 

Note that the Action will operate under the Permissions authorities of who created the Action, so you may want to consider setting up a userid owned by the IT department to be used for Actions and also any Sync Table connections.

Thanks a lot! This is very useful. We started to experiment with that solution and your additional information here will allow us to perfect that idea.