Assuming you are on C.05, in order to install 2008 you should wipe everything again and then procceed flashing.
Download C.05 from herehttps://realmeupdater.com/downloads/archive/RMX1971EX/
You cant install 2008 when you are on 1805.
One is base on RUI and the other on COS. Clean install is required.
So, in short terms, you download C.05 and 2008, wipe everything, install C.05, wipe everything again, install 2008.
@der_stephan it's funny you are asking how to, since i followed your guide on 3rd page in order to install.
You MUST install C.05 on TWRP and NOT boot it. (the version of TWRP @der_stephanposted on his 1805 guide worked for me).
If you have flashed successfully C.05 and booted it, it may end up in boot loop if you try to flash 2008.
So install TWRP if it's not already installed (TWRP gets overwritten when you flash AND BOOT stock roms, that's what i can tell from my experience), then,
Wipe everything including internal storage
Flash C.05 and reboot to recovery, NOT SYSTEM! VERY IMPORTANT!!
Wipe everything again and procced flashing 2008 and BitGApps.
Between wipes, use pull commands on cmd to transfer the files from pc to phone.
If you end up in bootloop it is reccomended to format data and reboot again.
Thanks, pal. This is quite helpful.
What do you think? If I make a backup of internal storage, then I install 2008 like you said, then I boot 2008, then I boot back in TWRP and restore the backup of the internal storage. Do you think, this could work?