Originally Posted by jae2014
Yeah I want to be able to go back to stock for whenever I sell/trade the device. Also, I was thinking about cancelling my AT&T plan and migrating my phone over to a different carrier like straight talk. I haven't done that before so I don't know if switching to a different carrier becomes harder if I am on a custom rom.
The phone can never be returned to truly stock, in that once the bootloader is unlocked HTC will always have a record of that, linked to the phone's IMEI. Therefore, when selling or trading of the device, you should always disclose to potential buyers that the bootloader was unlocked. Although I agree even with this disclosure, some buyers may want the phone to be as stock as possible.
On some previous HTC devices, the SIM unlock code only worked on Sense based ROMs (but not AOSP) but it didn't matter if it was a custom or stock Sense ROM. I don't see any other reason why being on a custom ROM would affect you switching carriers. Its a smartphone, and works like any other regardless of whether its on a stock or custom ROM. In fact, being on a custom ROM may even make that experience easier, as custom ROMs tend to have multiple carrier APNs pre-configured; and also some features such as personal WiFi hotspot/tethering which are blocked by AT&T (without the proper data plan) are easier to get working on other carriers using a custom ROM.
In any case, the previously linked RUU is what you need to return to (close to) stock.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)