[X Play General] mxnd:
Hello everyone, I just upgraded my phone to Dual SIM Marshmallow yesterday while leaving the bootloader intact and without losing any data (as far as I can tell). Yes, this is a dirty flash operation, so proceed with caution.

First a little bit of gibberish: Because I got tired of waiting around for the European 6.0 update, and also having interest in the Dual SIM function anyway, I inspected the gpt.bin of both XT1562 RETEU and XT1562 RETASIA with gpt_parser.py

Turns out: the userdata partition wouldn't suffer from misalignments by flashing a different partition table:

India gpt.bin gpt_parser.py output:

[-] Partition 42
[-] Partition type GUID: 1B81E7E6-F50D-419B-A739-2AEEF8DA3335
=> Partition type: unknown partition, UNKNOWN
[-] Unique partition GUID: 86E734CA-D096-B657-DA2D-095F241A084A
[-] First LBA: 7634944
=> Disk Offset: 0xE9000000
[-] Last LBA: 7634943
=> Disk Offset: 0xE8FFFE00
[-] Attribute flags: 1152921504606847000, UNKNOWN
[-] Partition Name: userdata

Europe gpt.bin gpt_parser.py output:

[-] Partition 42
[-] Partition type GUID: 1B81E7E6-F50D-419B-A739-2AEEF8DA3335
=> Partition type: unknown partition, UNKNOWN
[-] Unique partition GUID: 0C44D2CF-AFB9-7EEE-2FAE-03E3E1D02EB8
[-] First LBA: 7634944
=> Disk Offset: 0xE9000000
[-] Last LBA: 7634943
=> Disk Offset: 0xE8FFFE00
[-] Attribute flags: 1152921504606847000, UNKNOWN
[-] Partition Name: userdata

If you're not scared by the above feel free to proceed. The following worked for me, keep in mind that I can't know for sure if this will work out with your setup, so if you follow these instruction, you do so at your own risk:

1. I started out with LPD23.118-10 XT1562 RETEU with locked bootloader - userdata was encrypted.

2. I flashed LPD23.118-10 XT1562 RETASIA (a.k.a. Indian, and the version number is important because the OTA upgrade will only work with that - there was a faster google drive link somewhere I can't find it, sorry, will update the post if someone sends me a link) firmware like described here, but note the crucial difference:

fastboot flash partition gpt.bin
fastboot flash bootloader bootloader.img
fastboot flash logo logo.bin
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img_sparsechunk.0
fastboot flash system system.img_sparsechunk.1
fastboot flash system system.img_sparsechunk.2
fastboot flash system system.img_sparsechunk.3
fastboot flash system system.img_sparsechunk.4
fastboot flash system system.img_sparsechunk.5
fastboot flash system system.img_sparsechunk.6
fastboot flash system system.img_sparsechunk.7
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
fastboot erase cache
fastboot reboot

fastboot erase userdata is missing before the reboot because we want to keep our current settings and apps data that can't be backed up (looking at you org.thoughtcrime.securesms)

3. Boot normally, etc. it will optimise some apps because cache was erased etc. Dual SIM should be enabled now!

4. If you want to pop into the Marshmallow world without waiting till you get OTA notifications, grab the update, place it in the root directory of your SD card, and check for system updates.

5. Wait a long time while it upgrades, boots and optimises.

6. Happily check out your Android version: 6.0 and System version: 25.11.18.lux_retasia_ds.retasiaall.en.03 reteu (yes, apparently it recognises that it's a reteu phone - a little scary but everything is fine for now)

7. ???

8. PROFIT

I already switched my data/voice plan around making use of the Dual SIM ability. It works great and now I pay a couple euros less while keeping my old number! Keep in mind though, you should use two different network providers, if the same provider sees an IMSI try to log in twice poop might get shagged up. (Also, if you use an MVNO, make sure the infrastructure providers they use are also different)

Play safe and happy (dirty) flashing

Edit: fixed link