Just to finish this up - this is the result of my effort:
A day after sending them my last message with quotes from this threads I got a reply. They didnt refer to my request to make the ticket public so I'll cut& paste again their message and my reply.
I wanted to know if their suggestion is general so I asked what they think I should post in this thread. I have got no answer to that.
Draw your own conclusions.

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Hi Pnina,

That's correct, if you want to do that, it would indeed cost you $200 more to go from
Complete to Infinite than buying Infinite directly. Likewise, it costs over $800 if you go from
Pioneer to Complete adding all the modules one by one

You are free to sell your copy of Complete to someone else for more than $400 if you like.
We're just offering to "buy it back" at $400 if you want to get Infinite.

BTW, the license of Infinite you get is a full license, it's not an upgrade! If we "buy back" your copy of Complete, you cannot keep it - just like if you were selling it to someone else.

We understand this is a big concern for you. Feel free to contact us again the day you want to move to Infinite and we will work something out for you.

Best regards.
Selena Porter

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Selina,
Thank you for offering to sort it out for me when the day comes that I want to upgrade from complete to infinity. (obviousely I'm assuming that will be better than your current offer of a $400 voucher) .As I was also concerned how it will work when version(s) 7.5 or 8 come out I will assume your offer to sort it out for me will cover that too.
So for myself there is nothing more I can do but trust you will be fair and reasonable when the time comes. (I have no reason to assume otherwise).
However as you havnt really answered in general any of the questions I and other users raised in the renderosity forum, I wonder what you think I should say now. I promised to come back with information about your response. I can and probably will just cut &paste your reply, but if you have a more general message I could convey that.
Thanks
Pnina

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/