I have to clarify, so a few questions on my part:

Is this a shower with a preformed or manufactured shower base, and then you want to cement board and tile the shower walls that surround that base?

If the base is a manufactured base, are you keeping the already installed base and just rebuilding the walls and adding a niche?

If you are gutting the entire shower and installing a new manufactured shower base, have you already purchased it? Can you yell me what kind so I'm sort of on the same page?

And just to clarify, the niche...are you installing a manufactured niche or are you building the niche from scratch? I'm guessing the latter. But guessing wrong gets me into trouble sometimes.

hi mongoct thanks for responding.

this is an existing shower with a preformed mud shower pan. we were simply going to rebuild the walls and re-tile the walls and floor - however, we just learned that the pan may be compromised so we were going to flood it to see if it leaks and then... but in speaking with the tile store - real tile store not hd - i was told that i could simply pull up all the floor tile and kerdi the shower pan, even putting the kerdi down the drain a bit, install a new kerdi drain and then tile over that. (the original membrane was put in under the shower pan not on top)

we are planning on building our own niche.

thanks

ok so...nope. thought this job was going to be easy and straight forward. i don't know why considering my builder was awful!

decided today to go with kerdi board instead. it's about $100 more than redgard but easier and faster. so great, yay can't wait to get started.
so today my son and i start to take out the shower floor, we thought it would be like those youtube videos. chisel the tile away from the base. nope!
as we start to lightly chisel at the tile the concrete underneath literally crumbles and falls apart in our hands. huge chunks come away with the tile until all that is left is the plastic membrane on top of the plywood. (haven't looked at the ply yet because i am almost certain it will also have to be replaced.)
you can tell the builder never let the concrete cure properly. honestly it was like sand in places.
so my job now includes a new shower pan. kerdi to the rescue once again. but now i also have to buy a kerdi drain.

so far this stupid shower is $300 over budget and i haven't even started it yet!

"as we start to lightly chisel at the tile the concrete underneath literally crumbles and falls apart in our hands...you can tell the builder never let the concrete cure properly."

That's not "concrete". It's "deck mud".

Deck mud is a very lean portland cement mixture, it's about five or six parts sand to one part of portland cement, half the amount of cement that's in concrete. It's made to be more sand so that any water that gets below your tile can percolate through the deck mud and eventually go out the secondary weep holes in your drain.

The membrane being flat on your subfloor plywood, that's a problem. That membrane should have been placed on a sloped layer of deck mud.

But enough about that.

The Kerdi Tray and Kerdi board, it's an excellent system when done correctly. Just follow the Schluter tutorials. Go step-by-step. Make sure you cover any fastener heads and board seams so the system as a whole is water-tight.

Your old plumbing might be a 1-1/2" drain. The Kerdi drain is 2". You can upgrade your waste plumbing to 2", or you can use an adapter to make the transition.

Also, match the Kerdi drain material to your waste pipe material; ABS or PVC.

Sorry in advance if too much info.

never to much info. thanks so much mongoct.

a few more questions:
do you recommend any special type of thinset mortar? the tile place recommended servolight. i wold prefer something i can use not only for the kerdi but also to adhere the tiles. - white subway, hex marble and glass
so i have to buy kerdi band for the edges or can i simply use left over membrane cut down?
does it matter if i tile the floor or walls first?

thanks so much