I do not play recorder, or harpsichord, but I have friends who do, as I'm a fan of Baroque music. Let me tell you what I do know.
A=440 and A=415 are exactly one half-step or 100 cents apart (relative to 12-tone equal temperament).
I don't have a definitive answer, but the problem is that neither the recorder nor the harpsichord are (typically) tuned in 12-tone equal temperament, so if you were playing a piece on your recorder in D at A=440, asking the harpsichordist tuned to A=415 to play transposed up a half step would not work, because even with the transposition, most of the pitches and intervals would not be properly in tune between the two instruments (especially if the harpsichord was tuned to a historical meantone tuning, as most would be).
I believe that you would have to ask the harpsichordist to physically slide their keyboard keys up a half-step across the strings (most harpsichords these days have moveable keyboard beds and blocks that enable this), and then the harpsichordist would have to re-tune each string on their entire instrument to the same tuning used by your recorder.
In my experience, professional harpsichordists are used to this sort of thing. They transpose and re-tune their instruments all the time, since harpsichords always go wildly out of tune whenever they are moved to a new space and/or when the temperature or humidity change quickly, as with central heating or central air conditioning systems in performance spaces. Most harpsichordists would be quite able to re-tune between a historical meantone tuning and 12-tone equal temperament; many these days carry electronic tuners that can accommodate this.
More to the point, though, you would be asking the other person to do a lot of work to accommodate you. As far as I know, professional recorder players usually have pairs of instruments, one built to play in A=415, and a similar one built to play in A=440, in order that they don't have to ask everybody else to re-tune to the recorder. In fact I have a friend who is a professional recorder and transverse flute player who also has historical recorders built to play in A=395 and A=465, so that she can play certain historical compositions in the tuning in which the composer originally wrote them (for instance, A=395 for Charpentier). It must be very expensive to own all these hand-built historical instruments, but early music performers tend to be fanatical about these matters. Now my friend's husband is a professional harpsichordist who can retune his entire instrument to any temperament called for with a single tuning fork and his ears; I'm sure they have many different schemes for working things out.
It's all very complicated, but that's what it takes, it seems, to play early music in the modern era.