I think it doesn't matter. I changed to EFI boot about 6 months ago as it was allegedly faster to boot but I noticed no difference.
Your problem is you are stuck in a sort of limbo. You have an almost UEFI disk layout but are booting legacy.
This was the other thread btw Cannot upgrade due to unsupported disk layout for UEFI firmware - Windows 10 Forums
I would pick one approach and stick to it. So assuming EUFI the steps would be:
1. Make sure you have a bootable USB with Windows 10 on (in case you end up needing to repair boot files).
2. Make a backup.
3. Convert your disk 0 from MBR to GPT using MiniTool Partition Wizard (this is quite easy - just click on the convert mbr to gpt button and you shouldn't lose any data).
EDIT This is the button I mean (but the other way around - mbr to gpt)... Then click "Apply"
4. Create the msr partition as described in this post How to recreate a Microsoft Reserved Partition for GPT volumes on AppAssure Agents (137589). For you the offset is 229-128 = 101 so you would enter
5. Change partition one (your 100 MB EFI partition) attributes to EFI
select disk 0
create partition msr offset=101
6.. Create boot files (from elevated command prompt not diskpart)
Then try to reboot and see if it works. If it does then you are running UEFI and the upgrade should work OK.
select disk 0
select partition 1
set id=c12a7328-f81f-11d2-ba4b-00a0c93ec93b override