I’ve done it. Our server and all but one of our clients have been dist-upgraded to Debian Stretch. The dist-upgrade went smoothly on all clients. The server was another matter. Oh, the dist-upgrade was smooth but web-applications were ripped by the migration from PHP 5 to PHP 7. It was trivial to convert my recipe application to PHP 7, just a handful of MySQL calls needed changing. phpBB, OTOH, does not support PHP 7 and since we rarely use it, I will just remove it. It was useful when I taught in schools but I don’t need it now in the era of smartphones in every pocket. People use FB or e-mail or “messaging” and carry on. Coppermine Photo Gallery has a double whammy. It’s no longer supported by anyone and so will not be upgraded by the FLOSS community, most likely. I have invested quite a bit of work annotating photos in the database so I don’t want to abandon CPG. I can put it in a virtual machine running Jessie forever. It’s on the LAN so security is not much of an issue. My local library of Gutenberg texts is another matter. The CGI script was written in PASCAL, so that’s not a problem but the SWISH-e PHP interface does not build against PHP 7. The SWISH-e plugin is ancient, about 2012, so it’s not clear whether it will ever work with PHP 7. I just don’t want to dig that deep. SWISH-e still works so I could rewrite everything in PASCAL and carry on, but I could also move this web-application to a virtual machine running PHP 5. This library also was very valuable when I taught in northern schools with shaky Internet connections but it’s less important now. I can also use SWISH-e from the commandline if necessary. phpMyAdmin worked smoothly. It’s from Debian’s repository, of course.
So, there were a few bumps in the road, mostly due to web-applications not from Debian’s repository, but all in all it was a very smooth transition to a long term supported release. I like it.