After 3 support tickets asking the same question and receiving the same lame answer, someone in Cashaa Support finally gave me an answer I could accept. Here is what they wrote:

"Wallet address  are not case sensitive don't worry about  it .
if  you still want to change your wallet  address please visit your cashaa account  to change your wallet  address."

Seem like problem is solved.  I am using a My Ether Wallet (MEW) address for my Cashaa backup account.

I used the MEW for depositing my funds in the ICO too.

I have another problem though. 

It seems I have stupidly given them the wrong phone number for my mobile phone.

I asked them how I could change the last digit by saying this:

"Hi, my phone number has been entered incorrectly (WRONG NUMBER - ( +61) xxx xxx xx3). I am so sorry. The correct mobile phone number should be ( +61) xxx xxx xx6. I am not able to receive OTP to the phone number in the Cashaa records. Please help me change the phone number to  ( +61) xxx xxx xx6

...and they responded with this message:

"sorry we can't do this its seems suspicious we have to check things please wait."

Anyway, I've tried to fix it by going into My Account and clicking on edit but you can't change your phone number in that area.