Sorry to post late to this thread but I only joined a couple of days ago.
I'm studying a course on cat behaviour and inappropriate toileting is the most common problem that leads people to call in a behaviourist.
First of all, take your cat to the vet to see if there is a medical problem. That should always be the first port of call if a cat changes their behaviour. Your cat might be pooping overing the edge of the box becase he or she associates it with pain from constipation or arthritis.
Given that your cat is still peeing in the box there could be a few reasons for his/her behaviour:
Your cat may be one of those that prefer seperate boxes for peeing and pooing.
Do you have any other cats? There may be issues with sharing a litter box. The rule of thumb is one litter tray per cat plus one. So if you have 2 cats, you need 3 trays, 3 cats would need 4 trays etc.
You cat may be finding the catsan too painful to squat on and dig in, especially if he or she tries to sit on the edge of the tray. You could try slowly changing to a fine-grain, clumping litter. I use Sancat Gold for my cat and he's a big cat but finds it soft enough, it has a nice smell and it clumps well. To change the litter, you could offer a new tray with the new litter in it and see if your cat prefers it immediately. If that doesn't work, add a handful of the new litter when you next clean out the tray. The following week, add two handfuls (using less of the old litter), and so on until the changeover is complete.
By spraying "deterent" around the tray, you won't put your cat off pooping outside the tray, he or she may even find a more "private" place to do so. To get rid of the smell, try Urine Off, if the other suggestions haven't worked.
I hope things get sorted out, please give an update 