I'm very interested in this, because I take care of this list/tutorial: http://forum.xda-developers.com/z3-compact/general/list-stock-firmwares-d5803-d5833-t2906706
If on z3c the downgrading process always wipes the data partition, I will correct my guide.
I've been taking care of a similar thread for Xperia T for ONE year and nobody said he had a problem with this method. So, I'm sure it's good for older xperia.
Now, it's possible that our beloved z3c has a different behaviour with Flashtool.
The problem is that if you say to someone to make a copy of his/her stuff and make a test and then report the feedback, you have a chance of 10% to get an answer (if you're lucky).
Nobody wants to test, never. We are always the same few people that lock, unlock, relock the bootloader, flash, downgrade, upgrade, wipe, make backup and REPORT.
And anyways, only one experience is never reliable.
So bad, it really makes me angry.
Nevertheless, I will test by myself and report.
@dillalade I'm not angry with you, I'm sure you need not to lose your data...
I didn't wipe doing downgrade and upgrade and i haven't got any big problem. I just had a little problem downgrading to .98 , had force closes of android.process.media and android.process.acore , i solved it cancelling the data of applications: Contacts, Contacts storage (not sure how it's translated) and for the process.media just deleted data of Media Storage. Other than these the phone is like always
Upgrade didn't present also problems unchecking the wipes.
You can do all this without lose your data , don't worry.