The Message of the Day (motd) can be changed in the server.properties file. The line you're looking for should be at the bottom: motd=A Minecraft Server. Just change that to whatever you want your message to be, though there seems to be a limit of 59 characters according to the wiki. You can also change the formatting using some standard formatting codes.
I had the same issue when Minecraft migrated to Mojang global service a few years ago.
The fact is you have two possible logins for Minecraft:
The first one is on the Minecraft website.
The second one is on the Mojang website.
Try connecting to the Minecraft website first, make sur all information are valid (try to connect using user name, then try to connect using email). (I don't remember if there was an option to migrate from the Minecraft website so if someone remember feel to to edit.)
If both worked fine, then try to connect on Mojang website using the email or the username. The login is case sensitive so be sure you did not forget that on the username.
If you still can't connect to the Mojang website:
Try creating another account on the Mojang website (you do not need to buy Minecraft again), then try to add you Minecraft information with that account (in the profile settings).
If both of this did not work, then you should contact the customer service.
The work around I gave you was the manipulation the customer service made me do when I contacted them 3 years ago because I had an issue with the migration too.
Best Answer
As of December 1st, you will now use your Microsoft account to log in and buy.
For registration, simply click the green button in the centre of the page.
On the next screen, click "Computer", your OS, then "Minecraft: Java Edition" (if you selected Windows). You should then get:
Then press the Buy button. You'll be directed to create a Microsoft account if you do not have one, or you'll be asked to log in. After logging in, you will be directed to the purchase store.