It's definitely a good idea to ask what the options are if/when the current treatment doesn't work so well anymore. Might be worth asking both your oncologist and a second opinion.

They probably didn't test for HER2 on your original pathology report, but they can go back and test for that, or any other protein that's a target of a specific drug, any time they want. Tests like that ("immunohistochemistry" or "IHC") are inexpensive and take a couple days. If the next plan is one of these targeted drugs, they can probably order those tests now to see which drug might work for you.

You could also ask if they can do a DNA sequencing test on your tumor, which might suggest clinical trial possibilities. Sequencing is expensive and takes longer, but should be covered by insurance in your situation. (I just had a biopsy on one of my metastatic tumors so that its DNA can be sequenced. Hope it works and there aren't any insurance snafus.)