FYI - the conversion of outstanding bridge notes into the new preferred stock at the new issue price is very standard. Essentially, the existing investors put in money not yet knowing how the new shares would be priced, then the notes are converted into the new preferred when that price is determined. This will definitely be a down round (the Series D issue price will be much less than the Series C).
Also, an LOI is a good sign and a bit more than financing simply still being negotiated, assuming the new investors are actually parties to the LOI. The existing investors don't need to undertake due diligence, and the new investors have probably already been looking into the company. When deals fall apart, it's usually because due diligence reveals things the investors don't like. Still, these deals take time and I wouldn't expect this to close by Apr. 15.