This kindof sucks. I bought my 1D Mark II refurbished in 2005, it's been my backup since 2010 and I still have it, works great. Bought a used 1D Mark III in 2010 which is why my Mark II became a backup. So, I've been using the 1D series for nearly 8 years. Here is my dilemma.

The guy who sold me the Mark III will be selling his Mark IV but it would probably cost me as much as a new or slightly used 5D Mark III which doesn't have the 1D build, but does have cleaner high ISO, which is why I really want to upgrade anyhow. The 1DX I can't justify the expense of it, and I'd be worried about being able to get used to the 5D Mark III after years of the 1D's. Though it is looking like the logical option. Going from that to my 1D Mark III which would be my backup would have different interfaces and may be tough to go between the two. Anyone experience that transition between the 1D bodies and the 5D Mark III?

I have never done the transition you're asking about, but I still use both a Canon body and a Nikon body and switch between the two often.  They feel quite different in my hands (my Nikon is bigger and heavier than my Canon) and I find that I don't really need to think about which body I am using.   I am familiar with both and switching back and forth does not create any issues/confusion or slower operation.  Maybe it would be worse if the two bodies were more alike...I don't know.

I made the switch from a 1D Mark II to a 40D after shooting with the 1D for about 6 years. Got used to it pretty quickly. Sure it doesn't have the build or the high frame rate of the 1 series but I enjoy carrying a MUCH lighter camera around.

I think the hardest problem you'll have is learning the new features. The new menu system is better than the old one and the significantly larger LCD makes it easier to read the menu items.

Make the switch you won't regret it.