That's exactly what I was thinking about. Having the first round post-YC funding, trigger the conversion. That way, the sweetner is that if I am accepted, and it so happens that the w11 rounds are anything like S10 (i.e. largely convertible notes), then it works out better for them then because they would be converting to equity while the later stage investors are just getting their notes.
I was trying to avoid running up large legal bills, for the amount considered.
The investor can be considered 'friends and family'. I was thinking that maybe we do up a general agreement on some terms - i.e. everything that a 'standard convertible note' would have with a few tweaks. Then if I am accepted into YC, I then formalize it then - with the documentation that you guys have.
Would that suffice, or would you still want me to do everything with an attorney first?
As an aside, I am not in the US - so doing the official legal documentation and incorporating up there might be more tricky, if I were to do everything on my own (which is why I was thinking of a general agreement with acceptable terms on both sides - with specifics to be hashed out later upon incorporation).