Few random things for you, mostly not ISPConfig specific but migration in general..
- Do you have complete access to the source server? Are the sites flat-file based or sql-based, eg CMS system or similar? A mixture?
- You may be able to tar or gzip the web contents on the original server, unpack to temporary folder on destination server, chmod as appropriate then move into right place.
- If using some kind of CMS, check to see if its got a backup/restore database function inside it - you may get good results using first the ISPConfig APS installer to deploy the CMS framework (if available) then restoring portal contents accordingly.
- Perhaps create yourself some kind of a checklist for the domains - discover if any require particular PEAR or PHP modules that may not be on the server by default? Or any particular libraries that need installing. Last thing you want is to move a site and discover you've lost functionality because of XYZ library missing.
- You will also have to think about dns entries - if you have to change addresses for A records eg. www.yourdomain.com
or MX records for email.
Its as well a few days before you start moving anything to reduce TTL values to quite low, perhaps 600 seconds so when you make your dns changes you can guarantee uniform within a short period and no stale caches.
- Perhaps think about email for each site as well. What are you running on old and will it be same as new? Maildir format or mbox? Will you have to convert? Will mail clients have to be changed for any new settings?
- regarding apache or nginx thats only something you can decide based upon site requirements. nginx is known to have a smaller memory footprint and in certain circumstances outperform apache.
may give you an insight?
- You may find a dummy domain useful to point to new ISPConfig server in order test and check a migration strategy from old to new and highlight areas you need to address.
Good luck with your migration to ISPConfig - it will be worth it.