I didn't receive any answer here. Look's like my question didn't had enough research to deserve an answer. Here are some updates:
- I posted the same question on OnePlus forums (I hang around) and received results here.
- I looked for terms
downgradeon Yureka Forums and out of 159 threads, didn't find any satisfactory result. I found only one argument worthy enough to be mentioned in step 3.
- A OnePlus forums user mentioned that the 64-bit bootloader could be incompatible with 32-bit Kitkat in the event of downgrading, thereby, resulting a hard-brick. A Yureka forums user mentioned similar thing here.
- As per this guide on XDA by a known member of OnePlus forums, and this info of all the partitions in a device here, I can conclude that a fastboot type ROM can flash any bootloader and similar partitions required at first stage of booting.
- I read about bootloader and the name of the involved partitions from here and here.
A flashable ROM zip through recovery can hard-brick which is understandable here. However, if a fastboot type 32-bit Kitkat ROM is flashed on Yureka device running 64-bit Lollipop, should it be bricking it?
Note: This question is not meant to be open-ended, inviting discussion/opinion or chatty in nature. My question is only to know : Is there really a technical reason behind the warning or not?
There is a post on Yu forums by a Community Manger here which says in warning the following:
Once device is updated with Lollipop (64-bit), downgrading to Kitkat (32-bit) is not supported and may hard-brick the device!
The users on the forum seems to follow the same warning whenever someones creates a thread regarding downgrade to Kitkat. The same warning can be seen on this website too.
However, nowhere it seems anyone cared to mention the reason for such hardbrick. May I know why 64-bit to 32-bit is a nightmare for this phone, or for that matter, any phone? Could it be just a gimmick to keep users at latest OS?