This is my first post. I was on AT&T for a few years, but then decided to switch to Cricket mid 2014. I bought a new HTC One M8 AT&T new and off contract from an ebay seller last year. I got it off contract so I was able to take it to AT&T to get a sim card and activate it on my service at that time. Then I used their unlock service to get it carrier unlocked since I was not on contract anymore.
After I got it carrier unlocked, I ordered my cricket sim and it was super easy to port my number and get my new phone on Cricket service. Had been working great for more than a year. However, after Lollipop came out and it was announced that the HTC One M8 would get it soon, I kept waiting.... Then I realized that I am not going to get any OTA updates. Damn, I'm stuck on 4.4.2 using my AT&T branded HTC One M8 on Cricket. What to do?
I've had some limited experience rooting, recovering, and flashing my old Nexus 7 tablet. So I decided to research what I could do with my phone.
So the short story is. I am currently running the Skydragon 5.0.2 with Sense 7 ROM. It's running great! How did I get to this point?
Ok, so I had a little trial and error on some of my steps. But basically I was able to root (unlock the bootloader successfully), flash the firmware on my phone (was necessary to do otherwise installing any of the lollipop roms would not boot afterwards), install TWRP and make backups for recovery, and then flash the custom ROM.
More details: I tried to install the Skydragon before flashing the firmware and it got stuck booting. I flashed a new lollipop firmware version (probably the most scariest part. However if you try and flash the wrong firmware, it is supposed to fail.) Then after that installed the Insertcoin ROM. It worked great. After a couple of days I was reading their site and it looked like they are not going to develop further. I really liked all the tweaks available from Insertcoin, but was kind of discouraged to keep it because of maybe not future updates (marshmallow?). So I went ahead and installed SkyDragon and it works great. Another good thing about flashing the firmware and installing a custom ROM? No AT&T branding or bloatware at all. It's great. Oh, BTW, I was able to do this with S-ON.
I also learned to use TWRP to create backups. This easily allows you to try another ROM just for fun and then just recover your last backup.
Hope this helps anyone that is interested in doing this!