In the item master, you might witness values in the following fields to be frozen.
For an item, once stock ledger entry is created, values in these fields will be frozen. This is to prevent user from changing the value which can lead to mis-match of actual stock, and stock level in the system of an item.
For the serialized item, since its stock level is calculated based on the count of available Serial Nos., setting Item as non-serialized mid-way will break the sync, and item's stock level shown in the report will not be accurate, hence Has Serial No. field is frozen.
To make these fields editable once again, you should delete all the stock transactions made for this item. For the Serialized and Batch Item, you should also delete Serial No. and Batch No. record for this item.