How to alter widget type programmatically on a per-form basis?

Assume I have a user registration form with a select box widget. This select box should be shown in the user account, but on the user registration for, I need it to be a checkbox.

What can I do to alter the widget type for a specific form?
(For this case, assume my field can be select/checkbox built-in. It's just about switching between its existing widget types.)

Any suggestions would be welcome!