When a field is being referenced in a Flow you will not be able to change the Data Type and will receive an error, "Can't change the data type because it's referenced in the following Flows".
Steps to Reproduce:
Note: Deactivating the Flow will not resolve the issue.
This issue can be resolved only when the affected field is removed from the Flow.
NOTE: Deleting the entire Flow would also resolve the issue. However, to avoid re-creation of the complete Flow, you can follow the below-mentioned steps:
Now you will be able to change the Data Type of the affected field without any error.
Vote and comment on the Salesforce Idea - Ability to Change Field Type of Field Referenced in a Flow
Trailblazer Community - Cannot change the data type of field
Trailblazer Community - Changing the data type of a field
000387461

We use three kinds of cookies on our websites: required, functional, and advertising. You can choose whether functional and advertising cookies apply. Click on the different cookie categories to find out more about each category and to change the default settings.
Privacy Statement
Required cookies are necessary for basic website functionality. Some examples include: session cookies needed to transmit the website, authentication cookies, and security cookies.
Functional cookies enhance functions, performance, and services on the website. Some examples include: cookies used to analyze site traffic, cookies used for market research, and cookies used to display advertising that is not directed to a particular individual.
Advertising cookies track activity across websites in order to understand a viewer’s interests, and direct them specific marketing. Some examples include: cookies used for remarketing, or interest-based advertising.