once again I would like to ask for this...
my idea on how to implement it is easy... if you order a conversion, the system treats it the same way round it does for a new aircraft, but at the same time you order it, the old aircraft (the one converted) is destroyed and the data (life info) is inserted into the new aircraft plus the info, that it has been converted... e.g.
MD-81 -> MD-82
you pay something, and in the MD-81 info you can read (insted of "scrapped")
"converted to MD-82" (maybe you can add a link to the new aircraft... but doesn't have to be)
and the MD-82 get's the same age and landings etc. from the 81 but not a "aircraft constructed" but a "converted from MD-81" (and maybe also a link)
I don't think that's too complicated to build
and would allow to do the conversions of the 80s/90s (extra fuel tank, engines, whatever), the DC-8s, the DC-10/MD-10 (nobody ordered it for passenger aircrafts, but in this game we could ... and then add it to the MD-10/MD-11 fleet) ...
I don't know how many other conversins were available, but maybe you can invent some or give the option to re-engine the 707s like the KC-135 ... probably somebody would do it
or allow the conversion of an standard to an "ER" version of an aircraft... doesn't have to be that somebody did it in real life, but it's definitively possible to do it
ok... maybe it's not a very important feature and maybe only the MD-Jets have been converted very often... but I'd love to have it (as well as never built aircrafts like the MD-90-50)