Is there any method which force running aplication to reread DPI settings and redraw itself acording to new DPI?

I need this because my laptop LCD has 116 DPI and external LCD has only 96 DPI, so if I switch to external LCD after starting up some aplications, these aplications have too big fonts and icons.

Newly started applications use correct DPI settings and looks normal.

I am using KDE 3.5.10.

I have exactly same problem. Is there any progress in this issue?
For example on cinnamon I can change font scale for some app(vivaldi, guake, file manager, all cinnamon dialogs, etc) on the fly and some apps requests it restart(double commander). Whether it is possible such to implement in kde?

In Plasma 5.11 they made a number of changes to address DPI configs, including supporting multiple screens with different DPI settings (listed under Wayland support, but iirc, the DPI stuff should apply to both Wayland and X).
I'm not sure about per-app or per-window settings, though.

wayland per monitor dpi settings support only integer values(x2, x3, etc). If I need 125/130% scale I must use DE font scaling mechanism. Some DE already support changing scale on the fly(XFCE, Cinnamon, Budge(use wayland))