Retrofit complete! I have "the button" now
Replacement HEPA filter is $90 (pair), and I was told it can be replaced every 2 years.
However, replacing it does not seem easy. The
frunk lid overlaps the HEPA filter assembly, so the filter cannot be easily accessed as the non-HEPA version.
You either have to
remove the frunk, or
remove the windshield washer tank. Then lift up the entire HEPA assembly out, to dismantle it to get to the HEPA filters inside. There are also 10 bolts on the HEPA assembly that needs to be removed, which keep the entire box well-sealed. Tesla has
instructions for this. My technician recommended I make a service visit to replace the HEPA filter, rather than DIY.
Also, because of the new HEPA filter up front, I was also told that the
cabin filters ($17 each) can be replaced every 2 years, instead of every year. They will get less dirty because the air is already pre-filtered by HEPA.