Alright, speaking as someone who is currently in reboot land! I got through 8 chapters of my comic before starting it over and beginning what's on Tapastic today. I'm now about half-way caught up to where I left off in the original.
And despite what everyone here is saying, I'm SO GLAD I DID.
Now, I do agree with the above comments that a reboot should only be necessary when you're changing so much that it's practically a different comic. In my own case, my comic is essentially the same. Same characters, same setting, same story, just told and shown much better than the original. I changed things around, moved character reveals til later. For example, my comic takes place in a world where everyone wears masks. Face reveals are suppose to be a big deal in my story and in the original version, I showed the main character's face in the very first page. This time around, I held it off until chapter 4 so I (hope) it made for a much stronger impact.
In addition to that, there were some drastic changes that NEEDED to be made. For example...I was probably losing hundreds of potential readers because my comic was written right to left because...idk, I started it in high school and it was a "manga!!!!1!"
This time around, it's written left to right and I'm sure my few old readers from the original version appreciate that. :'D
I think another good time to start a reboot is when you're trying to have a "fresh start." My original comic sort of crumbled towards the end. I wasn't updating frequently, my readers were slowly dying off (it was posted on DeviantArt originally which is dying a slow death. I mean to give you an example, I use to get around 100 favs per page, now I get about 9). Everything my comic was sort of grounded on was kind of dying. I had done so many rewinds and edits that I'm sure people that were reading every page (all 15 left on DeviantArt) were confused as hell and it just got to a point that it was easier to go "Okay guys. I'm starting this over and doing it right this time."
Keep in mind, this was a HELL of a time pass. I started my comic in high school and started the reboot after college.
The reboot revived my passion for my dying project and it showed in my new pages which I think my remaining readers saw too.
I think that's another time a reboot is acceptable: If it revives your passion. Continuing the old version of my comic would have been beating a dead horse that was dead because I didn't take care of it. The reboot is a new horse that I'm gonna take much better care of after learning my mistakes with the old horse. In the end, it's gonna be a stronger and healthier horse than the last one.
And while my old readers are still waiting for things to catch up to where the old version ended, I think most of them are happy to see it at all. Plus with a rewrite, they are seeing some new content as well as just better presented content. The old version is so old now that they get some nostalgia trips when old parts come up but at the same time they're excited when new things come around. And on top of that, my passion for my comic is so rejuvenated that for the first time ever, I'm actually capable of sticking to a schedule of two pages a week, versus when my old version was ending and my passion was dying and they were lucky to get one page a month.
Sorry for kind of rambling here, but as someone who is basically working on a giant reboot and still will be for 4 more chapters, I kinda feel the need to defend them. Now once again, I agree that I don't think they're always the right thing to do. If you ARE gonna make a reboot though, really do make it something fresh. Take a year off or something to make the proper planning you need to in order to make an actually SUPERIOR comic (Okay, a year might not be necessary for everyone). Only do it if you truly believe you're gonna come out with something better in the end, something that's better in more ways than just the art. If you really feel like you're getting no where with your comic right now, the readers that do stick around will like it more in the end. If your passion is dying for your current comic, it will show and your readers will know. A reboot should be a fresh start with a revived passion in my opinion. And if rebooting revives that passion, it will show, and your readers will see it and enjoy the comic even more.