Interesting! A local company tried to hire me months ago to migrate a 2.6 legacy system to bleeding edge 2.7, I wonder if they have convinced someone yet.

I would have actually enjoyed that, the challenge being to use enough hacks and interfaces (six, __future__, etc) to make the subsequent, inevitable move to 3 a well-paid but trivial task...

You should ask to talk to a developer before accepting an obviously complicated migration. Ask about the numbers since they're not IP/Properitary. How many lines of code? How many functions? How many standard modules? How many custom libraries? Is everything Python? Are other languages being used too? That way you will know whether or not they are intentionally hiding the real issues from you to get you to do free work.

Unit test coverage ...

A bit off topic, but unit test coverage trivializes the work that goes into conducting a migration. Unit test coverage really only pertains to upgrades. A migration is not synonymous with an upgrade. Migrations are serious work that typically span entire ecosystems.

When I mentioned "the numbers" I wasn't hinting at unit test coverage. I was giving examples as to what the numbers could be. Obviously OP should ask lots of questions about all of the software running on a legacy kernel.

Were there any backwards incompatible changes in 2.7? I'm trying to remember if I even noticed when that version bump happened...

Yes, try to install it on a CentOS6 box as /usr/bin/python and suddenly no more yum etc. will work.

2.7 was essentially a bridge to Python 3. All features it has were backported from 3. This is why so many people before 2015 were saying that Python 3 doesn't provide any new features.

Release notes: http://hg.python.org/cpython/raw-file/v2.7/Misc/NEWS

TBH I think the main thing about that release, for me, was the introduction of context managers ("with"). I don't remember any particular breakage but I wasn't doing sysadmin at the time, maybe there is something in the bowels of redhat that relied on some broken behaviour...

I actually rejected it because I thought it would be extremely boring after a few days, I did that for a couple of projects when I was writing desktop apps in Python 2.x and I found it very easy (2.7 to 3.x is not especially hard either in most cases).

I never understood the point of hiring a full team for at least 6 months just for that. The only explanation I can find is that the project manager was clueless about Python.