Boozyuzi,
Personally I wouldn't use the 5005 centre with the KHT2005 system.
In the ideal system all the speakers at the front should be identical as they all seemlessly integrate together to form one big soundstage.
I pressume you wish to change to the 5005 to get a bit more bass with a perception that the 5005 is a higher quality speaker.
In my experience the KHT5005 has a slightly different character to the 2005 - slightly smoother and more laid back (although possibly more refined).
IMHO I believe you would do better sticking with the KHT2005 and adding an additional 2005 sat.
Also if you have the original Mk1 eggs then it might be an idea to invest in a better sub as this was always the weakness in the original design.
Good luck.