I haven't done too many G9 to G8 conversions. I've mainly just been using clones to convert and fit any G9 clothes to G8 figures:
Doing this only once has enabled me to fit any G9 clothing and accessories to G8
And if I don't want to trigger auto-fit to break the morphs I've been using this:
same principla, just putting G8 in the G9 pose.
As for converting textures I've only done 1-2 conversion and they haven't been seamless. I've used Wrap 4D for it and I can get things transferred over for the most part, but there are still noticable seems on the textures in some parts.
I have created some g8 to g9 and g8.1 to g9 morphs also so that you can fit g9 clothing and hair directly do genesis 8. Maybe it would be better to share some of those morphs? They've worked well enough for me anways. I haven't had too many issues with them. But if someone has already made morphs and shared them I won't repost what already exists here.