Opening up attic space
Modification of trusses is a tricky business. You DEFINITELY must not remove pieces without an engineering analysis of the effects on the truss. There are no simple formulas that I could or would give you to compute the effects. The reason is that removal of elements will convert the truss from a structure where each piece is either in tension or compression (no bending moment) to one in which individual pieces carry bending moment, which is much harder to analyze, and totally changes the capacity of the truss.
Conclusion: You need to hire a structural engineer to analyze the proposed changes. Alternatively you could replace all the trusses with a different type which better meets your space program. Or just leave them alone and live a long and happy life.