The old C to D conversion question

[Spike Kavalench]

Okay, I've read how easy it is to convert to the D model (stainless steel fuselage skin, drilled out main gear bleed orifice and reduced elevator travel), but if that's all there is to it, why does the Skyport catalogue refer to an "SMK-1A trim control conversion" and a "wide trim table plate"?  Is there a requirement to purchase these extras?   D.F. Spike Kavalench CF-JGW