No you cannot. You do not have an EP with you new company yet. All you have is the IPA (In Principle Approval) You do know what "In Principle" means, don't you? If for some strange reason you don't, it means only when you meet all the criteria will they issue you with an new EP. This could mean Medical checkup (doubtful if you are already on an EP) or your old employer hasn't released you yet. You cannot get an EP issued, nor a letter allowing you to work until it is issued, until the old employer has cancelled the EP for their job. Don't even think about it. Not unless you want a quick one way ticket to wherever you came from, on your own nickle.
You mean the starting date on the IPA? Or on your Employment Contract with the new employer? The starting date on the IPA may be able to be changed, but I cannot say for sure as I've never met anybody that shortsighted before. You can, however, pay you old employer salary in lieu for the one week short notice if that option was in your contract and they are willing to go with it (that is what I would suggest) otherwise, you might throw a spanner in the works to the point that the new employer says forget it, you're not as clued in as we thought and then you find yourself out of both jobs.
SOME PEOPLE TRY TO TURN BACK THEIR ODOMETERS. NOT ME. I WANT PEOPLE TO KNOW WHY I LOOK THIS WAY. I'VE TRAVELED A LONG WAY, AND SOME OF THE ROADS WEREN'T PAVED. ~ Will Rogers