Hide values in a lookup field for non-logged in users.

Hi all. I have a form that contains a lookup field. Let's call the form "Booking form". The lookup field is called "Package". Basically, it lets the customer choose the package they want to book. This field is a lookup to another form called "Packages" that is basically a list of available packages the customer can pick. Some are 1 hour, some 2, some 3.

Occasionally, we need to customise a package. Say the customer purchased a 2 hour package, but we decide to change it to 2.5 hours. But we don't want customers to be able to pick the 2.5 hour package when they fill out the form.

I have tried looking for ways (using Workflows and Deluge scripts) to hide certain packages from non-logged in users - basically the customers, but I can't find a way. I can hide specific fields easily, but not specific values in a field.

The other thought I had would be to allow logged in users to modify the value in the field after it has been submitted, but - being a lookup field - we can't modify a value without modifying the parent field.

Can anyone think of a way to achieve my goal or point me in the right direction? Any advice would be appreciated

Thanks in advance.