Hi there, we did this in May this year for approx. £550 additional cost for 2 adults and two children, We were staying onsite with free DP.
We went to Guest services in Downtown Disney and it was done on that days exchange rate. Before we gave the go ahead to convert to regular AP's we asked if they would be valid for 12 months from that day or the first day of our trip. When it all went through they had been activated from the day that we changed them to AP's not the first day of trip when we entered the parks, something to consider if you are working it around school holidays, it may work out better to chance it and convert on the last day. Obviously you wouldn't be able to take advantage of the store/restaurant discounts though throughout your trip.
Also we made sure that we used them before we left (even though we could have carried on using our key to the world cards), the reason being is that I had read that if you lose them it's far easier to replace them if they have already been activated, otherwise you may encounter a lot of problems getting them replaced - just something to consider if you do upgrade.
We upgraded to the AP as we had flights for Christmas already booked. When we went to book accommodation for this trip, we wanted to add the dining plan (worked out cheaper to do lots of special meals for the children). We were unable to do this through the UK site, they told us no dining plan could be added if we hadn't booked as a package with park tickets. We had to book through US site which worked out a couple of hundred pounds more, so it hasn't worked out quite as cheaply as we originally thought!!
Hope some of this helps.