Django change model fields attributes?
If I change the model fields attributes in django should I remigrate them? 

Yes of course. Until you're willing to throw away your whole existing database,  that is.

will it afftect the data that is already stored there?

Depends on your exact changes (some field attributes like blank or editable won't impact the db schema at all, only the Django code).
Now if your schema changes are to affect your data, you can write data migrations too  

is it safe to make that change in an already running project?

Well that's what migrations (schema and data migrations) are for: having a safe, reliable, repetible, testable way to alter your db schema.
If I change the model fields attributes in django should I remigrate them? will it afftect the data that is already stored there? is it safe to make that change in an already running project?