However, I have successfully upgraded quite old Redmine systems in the past. For me it was a matter of starting with a backup of the current system, and the latest TurnKey appliance. Probably the most important components are a good backup, plenty of patience, access to Google and a childlike willingness to "see what happens if ...".

For starters, I'll note that the current v14.2 TurnKey Redmine appliance is a bit dated. We have been working away in the background to get our library updated, but it's taken much longer than anticipated. We hope to start releasing our next release (v15.0) next week (fingers crossed) but Redmine won't be ready for another few weeks yet I suspect. Regardless, the current appliance should still suit your purposes. It still receives OS security updates (for another ~2 years) and should support the latest version of Redmine fine.

My above noted success started with a backup of the old Redmine (including the database). I suggest that you leave the old server as is and move your backup to a new Redmine server. That way there is no down time while you work on the upgrade so less people stressing at you. It also means that if things go totally pear shaped, you can trash the new server and start again. Depending on how much usage it gets, that may mean you'll need to replay the DB restore/upgrade step and transfer updated files, but so long as you take notes of what works (and what doesn't), that shouldn't be too arduous.

I used a TurnKey backup (TKLBAM), but rather than doing a full restore, I just restored the Redmine DB and then manually moved relevant Redmine files/directories. So a manual backup (and restore) of just the DB and Redmine files (e.g. like this) should also work fine.

From here on in, everything will be done on your new Redmine server. My first step would be to load the old database (how you do that will depend on how you did the backup). The next step would be to download the installed version of Redmine. That may also possibly require upgrade of some components to match the requirements.

Then you'll want to have a look at the Redmine upgrade doc page. As I've already covered the fiorst few bits, you can jump straight to "Step 3" (follow "Option 1").

From here on in, it should just be a case of following the doc and googling if/when you hit any issues. My guess is that most problems will relate to missing, or incorrect dependencies. So long as you have a compatible version of Ruby, they should be easy enough to work around.

If you do get stuck on any issues, or have any specific questions (that I've glossed over...), please ask. Even if I don't know the answer, my google-fu is pretty strong and can likely head you in the right direction. We also have a guy on the team who has more Ruby experience than me, so he may also be able to assist with any specifics that baffle me.

The last thing I'll say, is that I urge you to document as you go. Then, worst case scenario, if things get really messed up, you can bail and start again from scratch and will have some documentation to speed things up second time around.

Good luck with it and please let us know how things go! :)