HI is there a short guide somewhere
to modify my target config or/and kconfig to select a different SOC.
As an example SOC ar9340 as the soc for gl-ar150 instead of SOC ar9330 or ar9331 ?
(Moved to the For Developers section.)
There is no guide for that. You need to define low level hardware controls rather deeply.
Gl-ar150 has its chip drivers defined here:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/ar71xx/files/arch/mips/ath79/mach-gl-ar150.c;h=9febc7a839262254907f3aa31f4cff6ba94097b0;hb=HEAD
That is one of the places that you likely need to change
Edit
I noticed that you have another rather detailed thread about your trials. Based on that, my earlier advice above looks rather unnecessary. But still, there is no practical guide for you.