Please reopen this - looks like in vendor/magento/module-downloadable/Model/Product/TypeTransitionManager/Plugin/Downloadable.php:66 the code is expecting some data by the key 'is_delete' on the downloadable data / downloadable product link to be able to change from a downloadable product to virtual, however there is no way to make that data be there.

[ksenia-zlotin]

Preconditions (*)

1. Downloadable Product 1
   qty: 100500
   status: enabled, IN_STOCK
   weight: This item has no weight
   links: no links, just samples
   samples: 1 sample file

Steps to reproduce (*)

1. Log in as Admin
2. Open Downloadable Product 1
3. Uncheck "Is this downloadable Product?"
4. Save the product

Expected result (*)

1. Save is effective.
2. I can buy a virtual product with that SKU
3. References to samples are kept in the database but the type_id and/or switcher prevents using this information almost like the weight is kept in the Virtual type of product. this gives an ability to switch back from virtual to downloadable smoothly without losing data
4. Alternative: if the first 2 options cannot be implemented, then Merchant receives alert or validation with instruction to remove links and samples first and only then Magento application allows to uncheck "Is this downloadable Product?"
   Example:
   

Actual result (*)

1. Save is not effective.
2. When I try to add product through API the product is out of stock behaves like a product that is out of stock (missing links) so it is still downloadable.
3. No alert. No validation. The checkbox "Is this downloadable Product?" is checked again, however, it was unchecked before save.
   

[m2-assistant]

Hi @ksenia-zlotin. Thank you for your report.
To speed up processing of this issue, make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, Add a comment to the issue:

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[ksenia-zlotin]

@magento give me 2.4-develop instance

[magento-deployment-service]

Hi @ksenia-zlotin. Thank you for your request. I'm working on Magento instance for you.

[magento-deployment-service]

Hi @ksenia-zlotin, here is your Magento Instance: https://c01b5cfce204133f8c94beb26e5b07aa.instances-prod.magento-community.engineering
Admin access: https://c01b5cfce204133f8c94beb26e5b07aa.instances-prod.magento-community.engineering/admin_ea10
Login: 2399bb11
Password: 70c9dba9625c

[m2-assistant]

Hi @engcom-Bravo. Thank you for working on this issue.
In order to make sure that issue has enough information and ready for development, please read and check the following instruction: 👇

• 1. Verify that issue has all the required information. (Preconditions, Steps to reproduce, Expected result, Actual result).
• 2. Verify that issue has a meaningful description and provides enough information to reproduce the issue.
• 3. Add Area: XXXXX label to the ticket, indicating the functional areas it may be related to.
• 4. Verify that the issue is reproducible on 2.4-develop branch
  Details

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[engcom-Bravo]

@magento give me 2.4-develop instance

[magento-deployment-service]

Hi @engcom-Bravo. Thank you for your request. I'm working on Magento instance for you.

[magento-deployment-service]

Hi @engcom-Bravo, here is your Magento Instance: https://c01b5cfce204133f8c94beb26e5b07aa.instances-prod.magento-community.engineering
Admin access: https://c01b5cfce204133f8c94beb26e5b07aa.instances-prod.magento-community.engineering/admin_c589
Login: 940b12c7
Password: 355df5f56006

[engcom-Bravo]

Hi @ksenia-zlotin,

Thank you for reporting and collaboration.

Verified the issue on Magento 2.4-develop instance and the issue is reproducible.

Steps to reproduce

We have followed from here #24122

Screencast.from.08-02-24.02.27.10.PM.IST.webm

After unchecking the Is this downloadable Product and after saving it is showing checked Is this downloadable Product

Hence Confirming the issue.

Thanks.