Let me start by making sure we are both talking about the same product, you talked about breaking into The Beovision 6-23 & HD discussion, but in fact it was the Beocentre 6-23 & HD which was being discussed, whilst similar products, they are different.
Whilst I understand Joeyboygolf and beobird point about HD on a Beovision 6-23, your point is that you were told something which was not correct or not as you understood it, so you feel that you were not told the truth. To be honest, in the dealers defence, at that time everyone appear to be somewhat unsure of the situation with what products were HD ready and when they would be, but what are the facts as I see them:-
First your Beovision 6-26 Type 9252 is a Mark I Beovision 6-26 and I find it difficult to believe that it was HD ready because even by the time the Beocentre 6-26 was introduced the HD ready kit was apparently not available. Whatever the case you can check the status by going into the Customer Service Menu which I outlined in the other thread HERE
I purchased a Beocentre 6-23 in March 2007 which was NOT HD ready, as it was for my gym, it did not concern me. However my supplying dealer told me it would be HD ready, because that's what they had been assured by B&O.
Anyway, assuming when you check the Customer Service Menu it is not HD ready, which I expect to be the case, then to make your Beovision 6-26 HD ready you will need a Type 4371 upgrade kit which will have to be fitted by the dealer, clearly it is up to you if you wish to take this action, but at least you have the facts to come to your decision.