Changing the PK column of a data extension without completely recreating it

Your best bet is to do a 'Create from Existing' and choose the current DE.

This will prefill the field editing section with the existing fields/data types/etc. from your origianl DE.

You can then change your data type from number to text and then 'create'. From there you just import in the data into this new DE and you are good to go.

There really is no way to edit an existing DE's primary key, so recreating is the only way to do it.

It's kind of surprising that different parts of Marketing Cloud have a different way of doing things, so it will be important to follow the instructions mentioned here:

1. Go to Email Studio > Subscribers > Data extensions
2. You should see a list of data extensions with the left panel showing the folder tree (if not hidden) and the blue Create button in the top right corner
3. In the window select Standard Data Extension and click OK. In the first step called Properties select Create from Existing in the Creation Method dropdown.
4. Proceed to the 3rd step called Fields and configure the fields as you please (including changing the data type, primary keys and nullable settings).