Hi,
I don't think you will be able to perform a direct ugrade from 15.1X49-D110.4 to 18.4R3-S2. You are expected to see an error "unsupported h/w" if you try the upgrade.
Even as per the KB link you mentioned in the post, you can upgrade the box from 15.1X49-D110.4 to 18.2R3 or 18.2R3 based Service Releases directly and from there you can upgrade to 18.4R3-S2 without any issues.
So please follow the step upgrade and you should not see any issues:
15.1X49-D110.4 ---> 18.2R3 or 18.2R3 based Service Releases ----> 18.4R3-S2.
Hope this helps.
Thanks and Regards,
Pradeep Kumar M
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