Problem solved.

The solution that I came up with is to add another parameter and pass the label to this new parameter as the value, that way when it is used further on in the drill through reports it becomes the value and I no longer have to change the text. I couldn't change the original parameter as it is used in the SQL where clause and saved in the database as the letter and not the word.

Then I can simply use the expression:

=iif(Parameters!prm_freq_lab.Count > 3, "ALL", join(Parameters!prm_freq_lab.Value, " & "))