Users shouldn't be able to change the media source plugin after the media type is created

The source field type and also the provided metadata are very media source specific. Changing media source could potentially get you into all kinds of trouble, so +1 for not allowing this to be changed through the interface.

Changing the source will probably not be a 80% use case. If this really needs to be changed in some cases there could be a contrib module that supports this or you could write a custom update hook when needed.

+1 for preventing the change of a source type.

Is it possible to prevent this on API level as well?

Maybe we could implement MediaAccessControlHandler::checkFieldAccess for that

This needs some tests as well. Besides that, I'm not sure if we should use a disabled field or should just show a message. I think we should at least add a explanation that the source setting can not be changed.

Maybe we can add a message like the one you see when changing field storage settings:
"The media source impact the way that data is stored in the database and cannot be changed once data has been created."

+++ b/core/modules/media/src/MediaTypeForm.php
@@ -126,6 +126,9 @@ public function form(array $form, FormStateInterface $form_state) {
+      '#disabled' => !empty($source),

#5, not sure if we want to prevent this on API level, what would we gain from doing that? There could be use cases for this, especially when we starting adding different oEmbed based providers that all kind of add the same metadata.

@marcoscano, it would be nice to allow changes as long as you don't have media assets, but that could be a followup. This already fixes the most important issue. I just copied the message from the field storage settings form.

1. +++ b/core/modules/media/src/MediaTypeForm.php
   @@ -118,14 +118,23 @@ public function form(array $form, FormStateInterface $form_state) {
   +      $source_description = $this->t('<em>The source plugin has been set and cannot be changed after the media type is created.</em>');
   

   We should probably not use the term "plugin" here.

   So maybe:
   Media source cannot be changed after the media type is created.

   Terminology is not my strongest suit though.

2. +++ b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
   @@ -49,6 +49,10 @@ public function testMediaTypeCreationFormWithDefaultField() {
   +    $assert_session->pageTextContains('The source plugin has been set and cannot be changed after the media type is created.');
   

   Make sure we change the text here as well.

Looks good to me! Created #2928851: Allow the media source of a media type to be changed when creating a new media type as a followup.

Testbot hickup

Adding review credit for @seanB

Committed as b5b3949 and pushed to 8.5.x.

This can't be backported to 8.4.x because it has a new translatable string/string change