Rust: compare datetime with fixedoffset to datetime with timezone

How does one convert between DateTime<FixedOffset> and DateTime<Tz>, in order to subtract to get a duration, compare inequality, or reassign?

An attempt to do this directly yields the error:

mismatched types
expected struct `DateTime<FixedOffset>`
   found struct `DateTime<Tz>`rustcE0308

Obligatory MRE:

    let mut a = DateTime::parse_from_rfc3339("2022-06-01T10:00:00").unwrap();
    let b = a.with_timezone(&New_York);
    a = b

>Solution :

Convert the timezone of b into the timezone of a before assigning it:

a = b.with_timezone(&a.timezone());