I'm not sure tbh, when are you taking delivery? Do you have your VIN yet? If you already locked in with Tesla I think you'd have to call them and be like "no sorry going with another 3rd party - I want to XXX down can you generate a new MVPA"? They will then generate a new MVPA. Then apply for DCU loan (yea, you have to get referral and all that) and send DCU the MVPA to get approved there.
I'd just call Tesla Financing directly tbh... you can decide of the interest differential is worth it to switch over