Before moving on to examine the procedure I have adopted in this process, it is worth mentioning some issues related to the ethical and practical problems related to the modification of a “perfect” composition.
Ethically: can one give oneself the right to alter the form of a composition that Corelli has decided to transmit in a specific form to posterity? Practically: what shall the trumpets play as, obviously, they cannot play any violin part, as they are not chromatic instruments? 58
The first question rose, perhaps for the first time, by Simon Harris: “if Muffat's ‘Armonico Tributo’ really was modelled upon the Corelli concertos we know, why did Muffat write for a five-part string band and not a four-part one, and how is it that Muffat's concertos can be performed by virtually any combination of players between a trio sonata and a five-part string orchestra whereas Corelli's, apparently, cannot?” 59
As a matter of fact, there is plenty of evidence that the baroque orchestral instrumentation was flexible and that it was considered to be adjustable to the circumstances. Muffat Armonico Tributo's preface, as pointed by Harris, explains how a five part orchestral piece could be performed as written, or could either be played in the form of a trio sonata, or as a concerto in four parts or, finally, how it could be transformed into a concerto grosso by building two cori [two groups of instruments], respectively a concertino (two violins and bc) and a concerto grosso (the 5 voices of the original composition), and by deciding the moments in which the concerto grosso rests, leaving the concertino playing throughout. 60 A similar degree of flexibility is mentioned by Michele Mascitti in his "Concerti a 6" published in Paris in 1727. 61 An even better known example of a pre-existing composition whose instrumentation was expanded, is Francesco Geminiani’s Follia which consists of the transformation of Corelli's sonata op.5 n.12 for violin and continuo into a concerto grosso in 7 parts. 62
Overall, considering the history of Baroque music, the answer to the first question is: yes, we can modify - ethically speaking - the instrumentation of a baroque composition because that was common practice at that time.
The answer to the second question is more elaborate. Indeed, if the transformation of a 5-part piece into a concerto grosso in the way explained by Muffat does not require the composition of extra parts, the addition of trumpets to a movement originally written without them, poses a challenging problem as, as said above, a baroque trumpet cannot play any violin part, as it is not a chromatic instrument. Hence: what should the trumpets play?