Okey I see that there is no way out of this. About my spelling, yes I am not the best speller in the world but I am doing my best. I think it`s not so bad concidering I am from Norway.
Okey back to business. The reason why you should not perform an In-place upgrade is as follows: (1) If a company demands zero downtime you are in trouble (bhb3881dmb said he had one night to do this) if something should go wrong. Suppose you came to the last part of the upgrade and something happend you have to do it all over againg.(2) You are also faced with the possibility that the two versions get in conflict with eachother even thought you change the LDAP protocol from 389 til 390. (3) If you perform an in-place upgrade you also have to disable the exchange services, which can be difficult to start again if you do the smallest mistake. The time it takes to upgrade depends on the size and number of folders in the information store, and also server hardware performance. My statement were also based upon bhb3881dmb`s one night. If he doesn`t make it during the nigth because somehting has gone wrong, and f.eks 50 % if the users doesn`t have their mail. He would most likely have to look for another job. He also writes that he tried a in-place upgrade but due to "time constraints" he didn`t make it...aaaahhhhh. Luckely he has a job the next day.
I know that Microsoft says that an In place is a good solution, but preferable in smaller environments with a limited number of users and servers if you plan to use the same hardware. Even so you should not listen to all microsoft has to say, there are many things that are listed in the microsoft books that doesn`t work so good in work situations.
If you use the move mailboxs way you dont have to change the LDAP protocol and you can choose one way connection, I saw what you wrote about 2 way connection, that is actually wrong. I have done this in a company wiht 180 employes if I had choosen the two way connection it would have created to much traffic, and the "upgrade" would have gone much much slower. You can also use the swing method witch is a good alterenativ also. With the move mailboxs method you use exmerge to move useers mailboxes (or manualy, not preferred) or other programs like Nett-ÌQ. That is a program that makes you access mailboxes cross the servers. User can also use their mailboxes during the migration. Remember use the ADC connector and Xmerge on the exchange CD not the 2000 CD.
Also remeber to change the mx-record in dns/firewall config. if you change servers. And LMhost if you use those. THis was writen a litle fast, hope it is enough for you.