We have to bear in mind that this is a 3rd party firmware, not an official firmware from Asus.
When a firmware (or any computer software) updates from one version to another, there are changes between them, and there are migrating attempts during the update process to migrate old configurations and settings to work with the new version. In some cases where migration is not so straightforward, there are even backward-compatibility tweaking so that certain configurations and settings remain in their old form, and workaround measures implemented so that the system can still work with the old form.
In official firmware, we expect the engineer to know inside-out of the differences between the old version and new version, and have every measures to make sure the new version works properly after the update. But for 3rd party firmware, where there are portion of closed-source components, the update might have broken points, such as configuration that suppose to be changed to new format, but not fully changed. These are "landmines" that might cause issues.
If you start over with a fresh firmware with totally new set of configuration and settings, it eliminate these kind of issues.
However, you can still try your luck, and perform the "dirty upgrade" just like many of us. As long as it works well, then it is not necessary to reset and start over all the tedious config. You only need to factory reset if there is something wrong after the update. This is to test that does the same issue still occur even after the factory reset? If it still occur, then it could be a bug in the firmware that needs to be fixed, or it can also be a coincident hardware problem. Then, we wait and see if other people also facing the same problem. If only you facing the problem even after factory reset, then the problem is specific to your router, which could be hardware problem.
After performing a dirty upgrade, I always make a reboot after the system upgrade auto-reboot finished writing to the storage. This is to ensure that the system can survive the next reboot after upgrade without any issue, because after the upgrade auto-reboot, the firmware updating process might still be continued with its migrating and tweaking of config files. So, we need to make sure after all the final change, the system can still work properly without anything broken.
Some people recommended the very extreme way, such as pulling out the power cord, with the intention to 100% ensure that the old firmware and its configs are totally wiped out with no remains, and the new firmware you flash in will be like virgin to the router. Again, if you don't face any issue with dirty upgrade, you don't need to go to that extreme way.
When a firmware (or any computer software) updates from one version to another, there are changes between them, and there are migrating attempts during the update process to migrate old configurations and settings to work with the new version. In some cases where migration is not so straightforward, there are even backward-compatibility tweaking so that certain configurations and settings remain in their old form, and workaround measures implemented so that the system can still work with the old form.
In official firmware, we expect the engineer to know inside-out of the differences between the old version and new version, and have every measures to make sure the new version works properly after the update. But for 3rd party firmware, where there are portion of closed-source components, the update might have broken points, such as configuration that suppose to be changed to new format, but not fully changed. These are "landmines" that might cause issues.
If you start over with a fresh firmware with totally new set of configuration and settings, it eliminate these kind of issues.
However, you can still try your luck, and perform the "dirty upgrade" just like many of us. As long as it works well, then it is not necessary to reset and start over all the tedious config. You only need to factory reset if there is something wrong after the update. This is to test that does the same issue still occur even after the factory reset? If it still occur, then it could be a bug in the firmware that needs to be fixed, or it can also be a coincident hardware problem. Then, we wait and see if other people also facing the same problem. If only you facing the problem even after factory reset, then the problem is specific to your router, which could be hardware problem.
After performing a dirty upgrade, I always make a reboot after the system upgrade auto-reboot finished writing to the storage. This is to ensure that the system can survive the next reboot after upgrade without any issue, because after the upgrade auto-reboot, the firmware updating process might still be continued with its migrating and tweaking of config files. So, we need to make sure after all the final change, the system can still work properly without anything broken.
Some people recommended the very extreme way, such as pulling out the power cord, with the intention to 100% ensure that the old firmware and its configs are totally wiped out with no remains, and the new firmware you flash in will be like virgin to the router. Again, if you don't face any issue with dirty upgrade, you don't need to go to that extreme way.