halacha - What is the ruling for a bracha's usage being extended across different food forms?

There's a tricky halacha with brachos that I couldn't figure out.

It goes something like this: If you make a bracha on one food item without having others in mind, if more of the food item remains in front of you and more similar items are brought to the table the same bracha applies and needs not be repeated (e.g., chicken was on the table and fish was brought to the table, or apples was on the table and pears were brought to the table).

The exception to this rule is that if the food item changes form (solid -> liquid).

What exactly are the particulars of this exception and what should one do in the case where the original food item is gone as well?