They can only change the problem tyre. The new tyre won't be brand new it will be a used set from practice. I guess it is a small disadvantage for having to change.
In the '80s teams used to often run with different compound tyres on different corners. They sued to put harder wearing compound on the corner that took more load.
For full explanation the sporting regs can be found on the FIA site:
http://www.fia.com/resources/documen...ulations_a.pdf
Last weeks Autosport also ran a decent summary. Although the new one is out tomorrow.