What are the cons and pros of this change for users?
Particularly, how will this affect the 1BTC payout? And, just out of curiosity what is the reason for this change?
It doesn't affect the 1BTC payout, because as soon as the balance (the sum of the credit of all deposit) reaches at least 1BTC, it gets paid back.
Cons: almost nothing.
Pros: it's more fair for everyone.
Reason? If someone has a big account (example 1000 btc), and it gets completed, actually if it sends other 10 BTC, he will obtain results at the speed of 1010 BTC (now it doesn't matter, but as soon as hardware will start to fail, he would subtract hashing powers from new deposits). After the change, he will gets rewards at the 10BTC speed, so every deposit will have the ~10 months return.
The new feature will just "expire" old deposits when calculating the allocated MH/s per account.
However hardware will be kept at work for Pyramining, and the result is that there will be a better MH/s per BTC for everyone.