How to upgrade phpBB on 1and1

In the end of last year, I helped a charity society upgrade their forum from phpBB2 to phpBB 3. They host their web site and phpBB forum on 1and1 Business hosting plan.
I did have much experience on upgrading, updating phpBB etc. The procedures of upgrade from phpBB 2 to phpBB 3 are posted here, part 1 , part 2.
The situation on 1and1 is a little bit complicated. I need a bridge server to complete the upgrade.
Let me put the whole story in short.


There are three steps. ( I always use 3 steps to finish most of these kinds of job)
Step 1.
Backup everything, including database and physical files.
Move all above into a bridge server. I use Dreamhost as a bridge.
After this step, I have two forum, exactly same. One is on 1and1, the other is on Dreamhost.
Step 2.
Working on Dreamhost.
Upgrade this forum from phpBB 2 to phpBB 3, based on the procedures mentioned above, in part 1 and 2.
After this step, I have the phpBB 3 forum on Dreamhost, and a phpBB 2 old forum on 1and1.
Export the database from phpBB 3.
Step 3.
Working on 1and1
Make a fresh phpBB 3 installation.
Use phpmyadmin to transfer the database backup on step 2 into this fresh phpBB 3 forum
Then all done.
The reason why I need the bridge server is that 1and1 server has not enough resource to finish the upgrade. It is always gave 500 error. I think the memory limit works on it.