Sounds easy, probably isn't!  Each system requires somewhat different
nuances of the original expression to be discarded, depending on
the capabilities of the reproducing system.  These lost nuances can
never be recovered.

Doing a machine conversion between systems could be done generically:
convert the original to MIDI using an expression simulator, then create
coding for a new system.  Creating new roll expression coding requires
removing or approximating expression changes the system can't deliver,
so further degrades the already-degraded performance.

Simplifying music musically needs intelligent choice, i.e., a roll
editor who understands the system.  They can also tweak the underlying
performance to make it more suitable for the reproducing system.
Not easy to automate.

The end result of all reproducing systems is similar in that they
create expression by a sequence of suction levels consisting of
flat, slow and fast ramping sections.  Each system has a different
subset of controls, differing ramp speeds and differing-sized control
steps.  Broadly they fall into two categories, the bass/treble systems
(Welte, Ampico) and the theme/accompaniment systems (Duo-Art, Artrio,
Triphonola).

I suppose that coding a roll from a less complex system to a more
complex one of the same category might allow most of the original
nuances to be kept because few if any further compromises are needed,
but not the other way round.  Conversion between different categories
of system would involve notably more compromises and possibly unpicking
of any system-specific editing tweaks.

This can also be considered as the laws of physics in action: entropy,
or "randomness always increases" applies, so systems tend towards
disorder whenever they can, in this case poorer accuracy arising from
conversion.  Creating order (accuracy) requires work to be done, in
this case by a musician.

Julian Dyer