Some old mountain bikes don’t die, they just get converted. For example, you could turn your short-travel XC bike into a long-travel enduro ripper (risky) or use a built-in flip chip to turn your 29er into a mixed-wheel bike. To be clear, converting a bike is more involved (and more transformative!) than making upgrades like swapping tires or adding gears, and this survey is focused on the former.
Tell us why you decided to convert your bike — and why, and how — in the comments below.