Currently using FrameMaker v6.0 for many years to support a HUGE Users Manual with hundreds of screenshots. I am concerned with corrupting this monster document with an upgrade to V15. Any ideas if this will blow up?
Download the trial version and test it on a COPY of your FM6 version.
There really shouldn't be much of an issue, unless you have used specific codepage fonts for different languages. Starting with FM8, the character encoding base was switched over to unicode from the old FrameRoman encoding.
Also, if the screenshots have been imprted by copy rather than reference, you may eventualy run into issues (grey box in place of a graphic) regardless of which FM version that you are using.
?Images, probably not - fonts, maybe - going from FM6 to FM2015 crosses over the Unicode divide (FM8?). I'd advise you to take a back up copy & upgrade it on a non-production machine where you've downloaded the FM2015 trial & applied all the patches to it. Easiest way to tell what's going to happen...
re: going from FM6 to FM2015 crosses over the Unicode divide (FM8?)
Yep. FM8 and later don't just partially support Unicode, they apparently attempt to convert some character code points to Unicode. I've never seen any documentation on precisely what.
All of those characters are worth converting from CodePage mapped fonts to native Unicode (where the glyphs are populated, anyway). Perhaps the biggest issue in an FM6 to FM13 migration is having the legacy fonts on the new platform (esp. if the FM6 platform is Mac or Unix).
In my most recent full-time position, I made a habit of implementing all non-ASCII text as variables, so that when the shop moved beyond FM7 they would be easier to fix. Odds of you having a file with that forethought are about zero. More likely, they'll be just body text with a Character Format applied, or if you're really unlucky, just a font override.
Plan on lots of proofreading.
As Arnis suggests, keep a backup copy.
To minimize issues due to possible/incipient corruption pre-existing in the FM6 document, considering saving it to MIF in FM6, and then opening the MIF in FM13 (FM2015).