Changing basis of purchase taxes from purchase price to rendita catastale

I don't believe that you will be able to change your mind on this. The choice to opt for the taxation base being the catastal value has to be written into the atto, and since it clearly wasn't in your case then I think you are stuck with the purchase price as the taxation base. 

You paid tax on the purchase price because it was a unita colabenti - ie it wasnt a house / wasnt habitable.  Now it is a house, and will have a rendita.  However, the calculations were done then, and nothing can change that.  If the AdE wants you to pay back something then it will use the original calculations. 

I totally agree it seems unlikely...it's just I have heard of it being done but can't remember where or from whom I found out about it!

We recently bought a house in the province of Bologna and applied for residency. We discovered that because we took early retirement just over three years ago and do not yet receive a state pension or work we could not get form S1 to cover our health needs. This means that we have to get health insurance but this has not been straightforward. Please be aware if you are in this situation.  The commune have been very particular and have vetted every policy we have tried. We are very glad that we did not sign or pay without consulting the commune, who eventually gave us a list of their requirements to show to insurance brokers. We have ended up paying just over €2000 for both of us from Assitalia, who liaised directly with the commune for us. This was our cheapest quote except for one company who tried hard to sell us a policy which was for travel insurance and covered our health everywhere except Italy. We don't expect any responses to this post but thought our experience would be useful to others in a similar position.

It is a pity that you did not consult this site earlier. There is a wealth of information if you write "health cover" at the top of the page, your right hand side, and press the "search" button. One of our members, Penny, has become an expert on the topic.  You will also find lots of information on many other topics as well. And if you do not find what you want, simply ask a question. There are many helpful and knowledgeable members. In anny case, I hope that you will enjoy Italy and this Community. Welcome!