Thank you for your replies.
1. AFAIK, you can run two instances of Studio on two different computers now, even if they are connected into one and the same network. However, this limitation still holds true for previous Trados versions.
I had another talk with a SDL Trados representative today and I get different answers every time... This representative said that two Rreelance versions would not run together in a networked environment, only one at a time (like before). Two days ago I was told that it is possible... that's why actual practical experience would be preferred.
2. Once you have upgraded, you will have to return your T2006 license before you can activate the T2007 license. If you don't want to use T2006, you can use T2006 as long as you wish.
It seems to be still possible to use older versions such as 6.5 and 2006 with LIC files; only older 2007 versions/builds cannot be run any longer, though, just the latest one, which is ok. (at least that's what I was told today)
Allow me one question though: why would your client want you to work in an obsolete version? Moreover, there are only some functional improvements in T2007, the file formats should be fully compatible.
Many clients request to use a particular version of Trados and/or Word for their projects. Apparently they sometimes have problems with the file formats although there should not be any. This happened to us, too. While one TTX file could be edited in both versions without any problems, proper cleanup only worked with one particular version. Perhaps this is due to ini files etc. they have specifically created/implemented. Fact is, depending on the project I usually need all versions and have to switch back and forth according to the client's needs (from 6.5 up to 2007 and soon 2009). Many clients want bilingual files delivered (to update their own TMs).