I've never actually done a 1.2.x to 1.5.x upgrade in one go, but have done plenty of 1.3 to 1.5s.

As far as I recall, for Django itself, the 1.2 to 1.3, and 1.3 to 1.4 upgrades were pretty painless, but then 1.4 to 1.5 will involve some work if you make use of function based generic views, as that's when they were removed.

Worth looking at whether your third party apps are still maintained with the new versions as well, as you'll likely be on ancient versions of lots of them if you're using 1.2 (and some may even be obsolete). pip-tools is handy for this ( https://github.com/nvie/pip-tools ).