Uhm, no, actually I didn't 
(I simplified the story a bit) This is what I did:
First: letsencrypt certonly -d forum.mydomain.com --manual --preferred-challenges dns
And then (once the DNS pointed to the new server): sudo letsencrypt certonly -a webroot --webroot-path=/var/www/html -d mydomain.net -d www.mydomain.net -d forum.mydomain.net -d test.mydomain.net
(Reason: I already had the other certs on the new server so there was no need of including them in the manual DNS run.)
So this is not going to work?