Question: I have an Acer laptop (Aspire E1, x64, amd i7, ram 16, vga 2). 
My base tasks are using virtual machines. 
I use dual boot for `Win10` & `Debian 8.6`. The type of BIOS boot is `Legacy`.

I am gonna change my HDD to SSD and install just `Debian 9.8` (so need at least two primary partitions).

My mainboard supports both `Legacy` and `UEFI` mode.
I know `UEFI` mode just runs on GPT not MBR. 

Please consult, 

Do you agree to install debian in `UEFI` mode? 

If yes, is it enough to change BIOS type to `UEFI` and then start the installation? Or need more instructions?

My system specification:

```
Aspire E1-572PG - x64
BIOS Version/Date: Insyde Corp. V2.17, 02-Sep-14
Processor: Intel(R) Core(TM) i7-4500U CPU @ 1.80GHz, 2401 Mhz, 2 Core(s), 4 Logical Processor(s)
BIOS Mode now: Legacy

```

Answer: 
Debian 9 fully supports UEFI, but does not yet support Secure Boot, so you'll need to disable Secure Boot before running the Debian 9 installer. Unfortunately that is something you must do in the firmware settings ("BIOS settings"), and so is specific to your system's firmware implementation.

(Debian 10 apparently will support Secure Boot. Finding a way to support it in a way that would not compromise Debian's Free Software principles took a while, as an absolute requirement to rely on a binary blob that the user cannot reproduce from source at will was an anathema for Debian. A detached signature together with a reproducible build of the shim bootloader seems to be an acceptable solution.)
Changing to UEFI and starting installation will be fine. 
UEFI is the new normal, Legacy will go away over time. 

Latest releases of debian fully support UEFI

<https://wiki.debian.org/UEFI>