Easiest groups/lineages of machines to port software between?

[Dave Britten]

What programmable calculators have the advantage of being able to run programs written for previous or lesser-equipped contemporaries with little or no software changes required?

For instance, you could forward port something from the 41C to a 41CX or 41CV, and probably a 42S if external hardware or the clock isn't needed, and I think 11C to 15C would be a relatively easy conversion as well.

What about the 67? Does that translate forward to anything easily? (Excepting the hardware assistance from the card reader for the 41C.)

Then there's the 32S family, with a rather different approach to labels and register naming, so there's usually a good amount of overhaul to bring something over, but going from 32S to 32Sii to 33S to 35S is relatively painless.

And in TI land, you can go from the 58C to the 59 with no changes, and also to the 66 if the program doesn't use any ROM modules, and you can adapt any printer usage (watch out for the CE behavior difference though). But the 66 doesn't have enough memory to handle large 59 programs, nor does it have a card reader.

What other porting family trees are there? Mostly I want to have an idea what I've got the best shot of running something on if I find an interesting program on the museum DVDs or elsewhere.