Learn English – Professional term for “Apples to apples” comparison

Have you considered using the Dreyfus model of skill acquisition? In particular, his ranking is:

1. Novice
2. Advanced beginner
3. Competent
4. Proficient
5. Expert

Note that each level has a specific, quotable, empirically-driven definition that allows one to (more-or-less) unambiguously identify which category one belongs to.

That might be helpful if your application is a self-survey which is otherwise vulnerable to differences of interpretation: a standard model has the benefit of normalization (to a larger extent than, say, “on a scale of one to ten. . . .”).

That said, since perfect standardization is an impossible ideal, if you implement this, you might consider offering an "other" option.

Alternatively, the word specialist comes to mind.

I agree with Janus Bahs Jacquet's comment above that upsell (as normally used in the United States, anyway) refers to either (1) adding extra-price items to a preliminary order so as to increase the final price of the transaction, or (2) trying to persuade the customer to upgrade from the product model that he or she originally selected to a more expensive model. Neither effort is motivated by a sincere concern for the best interests of the customer.

If what you do is identify a need that the customer isn't fully (or at all) aware of and then sell a specially formulated service to satisfy that need, you might consider a lengthier term for it, such as "customized service to match the client's needs." In any case, I would stay away from upsell, even if the term were accurate.