I'm looking to flash a ROM's boot.img
on another ROM with TWRP 3.0. Is it possible?
If it's relevant, both ROMs are CM-based running Android 5.1 Lollipop.
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Sign up to join this communityBoot into TWRP, select Install and follow the screenshots for the rest.
(Click image to enlarge)
It's not a problem for TWRP to which ROM your particular kernel belongs. The size should not be greater than the partition. That's all.
If the kernel is not good for the ROM, your device would not boot into the ROM or would cause some other instability, so proceed with caution.