D1G is live on the new infra, thanks a lot everyone :).

The actual migration took a bit less than 10 minutes.

Also special thanks for the list rrq, it was very useful; it went down a
bit differently, here it is for posterity:

1. Preparation
  * Added /etc/nginx/sites-enabled/placeholder
  * in backup script:
    * rm /etc/nginx/sites-enabled/default
    * service nginx restart
    --> result is: after the backup, page is offline and placeholder

        in-place (hopefully)
    * This goes down at 19.00 UTC
  * on new nginx:
    galaxy.devuan.org / www.dev1galaxy.org redir to dev1galaxy.org
  * Forum only served at dev1galaxy.org

2. Actual migration
  * At 18.00 UTC we changed TTL down from 30 min to 5 min
  * At 19.00 UTC, after backup script runs (and page is locked):
    * transfer the 2017-08-10 backup to new site
    * DB restore
  * open the new site for changes
    * (X) iptables rules
    * (X) test with local hosts file

* Change A records to 91.134.216.234
* Increment TTL

3. with until DNS has taken effect (5Min * 2 = 10 Mins tops)

4. got to old server and block all tcp except 22 turn off services.
--
Evilham