The installer needs to know about other versions, so when a FRANKLIN(5) is released after a particular GNR(6) build - you can't upgrade from that 'newer' v5 to an 'older' v6.
I know it defies expected logic since it's a 'newer' v5 but schema wise it's not yet built into the 'older' v6's upgrade steps.
For exiting earlier:
Bug 42687 – ZCS install should identify and validate version from which upgraded & discussed in:
Bug 42609 – Upgrade from 5.0.19 to 6.0.0 or 6.0.1 fails - slapadd import failed around zimbraFeatureVoiceChangePinEnabled.
If you flip through the
announcements section you'll get a sense of release dates.
So the only 5 > 6 paths that are possible right now are:
5.0.19 & prior > 6.0.2+
5.0.18 & prior > 6.0.2 (& anything after 6.0.0RC1 but
not worth your time)
Thus you will have to wait till 5.0.21 & 6.0.4 since you can't upgrade major versions from a FRANKLIN(5) release that is 'newer' than an 'older' GNR(6) release. (Not gonna discuss possible hacks here.)
5.0.20 is 'newer' than 6.0.2 and actually still 'newer' than the upcoming 6.0.3 because its essentially 6.0.2 with security fixes.