Hi Mads,
Sorry this is a late reply I've only just started working this myself.

I recommend looking at this blog post by Stephan Gay from the HQ.

https://www.zpqrtbnk.net/posts/migrations-in-v8

Some things have changed since the blog post as it was written a few months before the official release of v8.

I found it to be very useful. Essentially it's a 3 step process:

1. Create a Migration which extends the MigrationBase class
2. Create a IComponent to run the Migration in a MigrationPlan & Upgrader during IComponent .Initialize().
3. Create a IComposer to add the IComponent to the composition.Components() so it will run.

Unlike v7 where there was separate umbracoMigrations table now the records of the migration get added to a new table umbracoKeyValue instead.

Again, I recommend looking at Stephan's blog post it goes into some of the details.

Some of the key differences are;

• There isn't an UmbracoUserComponent class, use IComponent instead.
• For the IComponent class inject the values in the constructor rather than at Initialize().

I hope this helps or you'll find it helpful in the future

Jamie

is working on a reply...