Assuming you''re talking about Direct3D here (the rest is much simpler since it hasn''t changed as much):

The whole enumeration and setup in DirectX 8 is much easier and cleaner. So ALL the enumeration and setup needs ripping out and rewriting - we found DX8 took roughly a quarter of the code and simplified other things.

As mentioned above all the DirectDraw stuff is gone - if you do any DirectDraw blit type stuff in the same frame as D3D stuff (which was a baaad thing to do even in DX7 in some cases), then that''ll have to be re-written.

Your flip chain/blt chain stuff needs re-doing.
Your texture creation stuff needs re-doing to an extent.

A better idea is probably to look at what you can keep and then re-write all the surrounding parts. The majority of your renderstates, texture stage state, vertex buffer create, draw*primitive, settexture etc calls should be pretty trivial to port.

A good place to look is in the docs under the "what''s changed in the API" topic and see how many of those things you''ll need to change.

Personally I''d re-write, and port just the core rendering calls, then even re-evaluate those to see how shaders can help.