Keep seat configuration of converted pax planes

[wilian.souza2]

I'm facing this problem when converting Tristars to the -250 variant: I lose the seat configuration of my used planes and have to spend almost 1M per aircraft just to get back the 1st class seats the conversion removed, as this variant by default doesn't have first class seats. I think we should at least be allowed to opt whether to keep current seating or not when converting pax planes to another pax variant!