1. No retro compatibility between 2.06 and 2.60 for Templates:
HTML flow of 2.60 has been rewritten to use Bootstrap natively. But it keeps the old 2.06 template engine, so you can't update all the HTML from the template itself. In LS3, the new template engine is more flexible, and you'll be able to change any part of the frontend HTML from templates.
2. No portability of workarounds
Workarounds have never been stable solutions, that's why it's called "workaround". Again, one of the main point of LS3 will be to make the workaround easy to share, publish, upload, with version control + possibility to add question advanced parameter by configuration (HTML + CSS + JS + XML)
3. Denis fork is a good one, but it's not in a LTS state.
Denis fork is interesting, and he continues improving the good old 2.06 with interesting commits. So, keep in mind his branch is not "LTS": he's still changing some core code not directly related to security issues.
en.wikipedia.org/wiki/Long-term_support
4. Holch approach is the best one
Keep your old surveys on the old engine, create the new ones with the new engine.