What material should I upgrade the teflon tube to?
The Ender 3 (original, pro, or v2) does not need any modifications/upgrades to print PETG. Temperatures of 230-235 °C are sufficient for PETG and well under the 250 °C that's not recommended to be exceeded on a machine with PTFE lining all the way to the hotend. PTFE doesn't melt above these temperatures, but it does start to break down and release chemicals that can be harmful. Note that teflon pans achieve temperatures comparable to or exceeding what you'd print PETG at all the time while cooking, and especially if you burn food. The smoke point of peanut oil is roughly the same as the temperature you print PETG at.
If you did want to change it (I wouldn't recommend this) there is no different tube you can &quot;upgrade&quot; to. Instead, users who want to be able to print higher temperatures switch to an &quot;all metal&quot; hotend, which means the tube doesn't extend through the heatbreak to the hotend, but instead stops in the cold part, with a metal passageway for the rest of the filament's path to the hotend/nozzle. You still use the same PTFE before that, though. All-metal hotends are not necessarily an upgrade; they're a tradeoff. The PTFE tube all the way through is a highly desirable feature because it has very low friction and no ridge/boundary between materials for semi-molten filament to get caught on when retracting and unretracting.