Disable certain fields in SharePoint list edit form

[Vaibhav_k]

Hi, how can the Period and Quartely values fields be disabled on sharepoint list edit form? 

I should still be able to add new fields in repeating table but for existing entries, it must show disabled.

I tried  in

DisplayMode= If(
Form1.Mode = FormMode.New,
DisplayMode.Edit,
DisplayMode.Disabled
)

But it disables the new rows as well. any suggestions?

[Vaibhav_k]

The fields have some default values so they are not completely blank.

 

[Vaibhav_k · Accepted solution]

If(
Form1.Mode =FormMode.Edit && ThisItem.ID>0,
DisplayMode.Disabled,
DisplayMode.Edit
)