What Is Their Main Difference?
'Used to' is a phrase, while, 'use to' is a regular verb. We use 'used to' to indicate a habit that was done repeatedly in the past and not anymore, while, 'use to' is in the present tense . Look at the following examples:
When one pronounces 'used to', the /d/ and /t/ sounds merge and sound like 'use to'.
Are They Interchangeable?
Their tenses and functions are different, so they cannot replace one another. Compare:
As you can see, this sentence is wrong.