switch to hardcore

in a datapack, is it possible to switch someone to hardcore mid-game?
I have tried this:
/data merge entity @s {playerGameType: 2}
but it says "unable to modify player data"
so yeah thats not gonna work,
So the reason im trying to switch someone to hardcore is because i want to change how their hp gui looks like, so if i cant use hardcore mode itself is there some other way? not using texturepacks