can i change tubes

What does CJ say?

It should not be a problem just make sure it isn't biased to hot. The 6550 and KT-88 are almost interchangeable the sonic are quite different.
I personally found glory in the reissue Gold Lion KT-88s. Use a good dealer like Jim McShane a sponsor on AA tubes forum, he is careful about matching and screening the tubes for losers. They cost a pretty penny for a non NOS tube, don't be shocked, but they spank everything else in the amp I am using. (Jadis DA-60 )

Below is text I got from CJ's FAQ page.

Questions about Conrad-Johnson Vacuum-Tube Components:

Q. Can I replace a tube with a different tube type?
A. No. You should only use the tube type that is specified by conrad-johnson or an exact substitute. Using a different tube type can be expected to degrade performance and may damage the product.

I would call or email them anyway. I have VTL 300 Deluxe and changed out the 6550's with KT-88's recently. The reissue Gold Lions KT-88's are as good as everyone is writing.

Good luck