Help: Coffee-maker, TCO (做咖啡壶的请进）

One of my supplier want to change thickness of fibre tube outside of TCO (Thermal cut off) of one Coffee maker, they want to use thicker tube instead of current one. This is one UL listed product. But our supplier stated that need not re-submit for agency testing after this change, though we insisted on.

Who can help me to persuade my supplier if u meet similar problem before? Thanks a lot.