Hi Paul,
That isn't an issue I have run into before and I just tried it out in a test app and was able to change the field type on my end. My suggestion would be if you are still having this issue this morning to reach out to the Care team so they can take a look at the field in question and see what might be going on. They can dig into it with you and take a closer look. You can submit a support case while signed into Quick Base by clicking on the question mark icon in the upper right hand corner and selecting 'Manage Support Case' or while signed into Community here by clicking on Support where it appears in the upper right hand corner. Definitely let me know if you have any trouble submitting a case.
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Evan Martinez
Community Marketing Manager
Quick Base
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