In fact you can't! But a workarround would be to play with the sub-from functionality that enable you to make appear or disappear fields on the form by inserting or not subforms. This feature is available from SP 18. You can find info on the "sd45_sp20_addendum_en.pdf" document (page 221).
By the way if it is related to access rights or roles you could foresee to use FOLDERS to only grant access to right persons when the status has changed. When the status change you change the Folder value and only the right persons are allowed to change the fields you try to protect.
First of all thanks for the responses My specific problem is this: I do have like 5 or 6 status per Service Call Tha last two status are "Completed" and "Closed". What we want is that our Help Desk people should be not able to change any field of the SC, when the status is "Completed", except for the status field itself, so they can change it from "Completed" to "Closed". If I use Folder Entitlements or Status Entitlements, these peolpe can not chenge the status from Completed to Closed, because they donÂ´t see the status "Closed", they cannot choose it, because is not appearing.
The only solution I see here is to entable the statuses before "completed" and "closed" that will have as effect that all fields are read only. You could use the smart action to trigger an SD_EVENT or SD_import that would change the Status from "completed" to "closed" in the background. Be carreful do not use sd_dataform it is executed with user rights and would not work in this case...