Stopping Italian residency and consequences for prima casa

I have been resident in Italy since March but now a job move will take us to France where we'll rent. I am assuming I can just email the commune to tell them I will no longer be resident but does it automatically follow that the Italian house won't be prima casa if we are renting in France? It's not a huge issue apart from the fact we are half way through renovating and are currently living in what will become an annex. What will be our main family home is currently derelict and when we resume restoration work in a few years this will become our prima casa. Hopefully we'll also be able to claim restoration tax relief for this, the biggest part of our total project. If we put the annex over to seconda casa status for now, when we return to Italy n a few years will we be able to simply flip the properties back over to prima casa status? Confused about how it works!

Oh, if life was so simple that a email would sort it.
You probabaly need to go and talk to someone there, and explain what will happen, as leaving it to an email will probably result in nothing happening at all.
Also where are you 'Fiscally Resident' as this makes a bit of difference, because if you are FR in Italy, you may not need to change anything.
My advice would be:-
a) have a word with the Comune, see what they advise (snigger).
b) have a word with your accountant here, tell him your likely scenario and see what he advises.
or lastly my personally preferred option.
c) ignore what everyone says, say nothing and carry on as normal when you return.

Flip's right, it's not that simplistic here. Firstly your prima casa for IMU is where you actually live, so the moment you tell the comune (in writing by recorded delivery) of your new address (remembering to claim the discount on your Refuse Tax if there is one) then the house becomes seconda casa for IMU and you pay at the higher rate.
If you claimed the prima casa tax discount when you bought the property then that cannot be changed. It remains.
There is no concept of swapping from prima casa to seconda casa and vice versa really. It is all about where your residency is.
When you come back and live in the annexe you change your residency back to the house and then there is no reason why you can't claim the tax credits on your renovation and claim the IMU discount and start paying tax etc.
As Flip says never send and email and expect anything to happen. Always, always send a letter recorded delivery and keep the receipt. Even if you go in person, if you haven't had to sign a form then follow it up in writing.

PS - give yourself lots of time to do this as there is a new law and many communications with your commune have to be done either via an online system or via a PEC email address and any attached documents will need a digital signature. Believe me, all those things take time to set up!

Also as a caveat, make sure the person/persons you liaise with at the Comune actually know about and understand what you're telling them, there can be a lot of, how can we say, 'enthusiastic amateurs' in the Comune Offices who rather than look stupid (which is not hard) tend to go along with what you say and reassure you all will be OK. That attitude cost a friend of mine a couple of €k in fines.....