In Raoul's original post on this thread, he stated the following:

"Soyuz TMM will have similar modifications plus a Regul satellite data relay system and a new Kurs-MM rendezvous system. Perhaps more significantly, it will be able to stay in space for up to 380 days, which will significantly reduce the number of Soyuz exchange flights (bad news for aspiring space tourists...)."

If these modifications work out, they could have two Soyuz lifeboats docked to the ISS and replace one of them every six months.

He also stated the following:

"We all know the Soyuz to fly in october will be an improved version (called Soyuz TMA) which will enable tall and short astronauts to fly and also remove the third cosmonaut from flying tasks."

The problem is that it takes a lot of training to fly a Soyuz capsule so you'd need 4 trained crew on the ISS at all times in case you had to completely evacuate the station. What happens if they're having to evacuate due to something like a fire where people could be injured or incapacited? You might not have enough people left over to fly both capsules down. Perhaps the Russians could do it by remote control like they did in their early days, or perhaps the flight controls could be made more automated so it wouldn't take much skill to bring them home. That was one of the central design goals of the CRV - fully automated operations by anyone. Simply strap in, close the hatch, and press a button or two and the vehicle would fly you safely to the ground.