Build after transition branch merged back in master...?

When the transition branch will be merged back in master, people who were building FFOS for one specific smartphone, what will change for them? [I’d like to motivate again Vosky who was doing great work for Nexus 5 on xda]

Then, will it be possible to update from a 2.5 spark build to this new build (2.6?) without loosing data? The problem is mostly for sms, we can back up the rest before the update. This question is for the time coming B2G will be again for daily usage.