With the exception of file modifications you won't lose any of your work. But there is a possibility of old custom templates being incompatible with the new version which may require you to manually update those templates after the upgrade. That's what ragtek is talking about.
If your not using the extra.css template for your template edits (pending what they are) I suggest using the extra.css template to minimise the need of potential lost work due to reverting. Also, the style properties or a large portion of css code can be placed inside the appropriate areas to further minimise lost work/template edits.
If you made it to the Master Template (this is not the default style but the core style), then yes it will.
Otherwise you wont unless you revert the template. I spent about two hours updating all my templates through reverting them (I have about 7 styles in use atm), its not that hard if you use notepad++ and compare the old template with the new to see the major changes
even after you update you will need to manually click a 'revert template' button to update any customised templates. they wont automatically overwrite, so you will have an opportunity to copy and paste the old ones somewhere to work out how to update the new ones.
in reality you may not need to revert them anyway. i have templates as old as beta 3 that i have never reverted.
If you do that then any changes to the template in the latest release will be lost.
All you need to do after an upgrade is the following:
Check for outdated templates - if there are any, it will only be templates which you have previously modified.
Make a note of any edits you have made to the now outdated templates.
Revert the templates
Reapply your edits.
I keep a list of my edits in a file so after an upgrade, any templates which are outdated I can easily refer to the file and reapply them.
Are you copying and pasting the same code back? Might as well not revert.
Or editing a reverted copy?
I copy my old template to disk.
Revert. Copy reverted file to disk.
Compare the two and apply edits to reverted file.
Doing that only sets the value of the template to new, but it can actually does more harm than good. When you revert the template, it adds the extra settings and template changes (such as, new hooks). If they decided to change something that was there in the beginning to something completely different and void using the old structure, then your site is going to look very weird.
You can also check for outdated templates form the acp in appearance if you wanted to manually check...but you will be directed there in the acp automatically when you go to appropriate section.
after upgrading the board all the templates that you customized will appear in the outdated templates ...
like was said before you can just copy and paste them to a text file on your local computer.
Revert the templates.
Then you can manually check the differences or use a document compare function in an editor if you have a lot of edits and just apply your edits in the same way you did before...Kinda wish you posted earlier before I upgraded so you could see as an example how simple it is from some screenshots.
Note to the next person upgrading and reading this...post some screenies if you can here for an example.