what about the light artillery ?
We will replace it with an
equivalent Profession as well.
(It is also a Cannon Unit.)
If we get rid of the distinction then artillery will be both city defenders and city attackers.
No, there will be
2 different Cannon Professions - not only 1.
Light Artillery --> Garrison Artillery Profession (Possible with normal Colonist and Expert Light Cannoneer that has special Promotions)
Siege Artillery --> Siege Artillery Profession (Possible with normal Colonist or Expert Siege Cannoneer that has special Promotions)
Royal Artillery --> Siege Artillery Profession (but King's Expert Royal Siege Cannoneer has more powerful Promotions)
Summary:
Instead of
3 Cannon Unit Types (Light Artillery, Siege Artillery and Royal Artillery)
we will have
2 Professions (Garrison Artillery, Siege Artillery- with
different Equipment requirements)
and 3 Experts (Expert Light Cannoneer, Expert Siege Cannoneer, Royal Siege Canonneer -
with different Promotions)
That is already all. 
UnitAIs can most likely be reused but might need to be
slightly adapted.
The main problem however will be to
adapt PlayerAI to equip enough of these Professions instead of building the Units.
(
Buying should not be a problem since AI can still buy the Experts. Proven in use already.)
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Everything that is a
"Cannon Type" of Unit
and can be build will be transformed into an equivalent Profession 1:1. (With Expert)
The main reason to do this is to get away with "I can happily build my land military (during WOI)"
What about the European Line Infantry unit, ...
Not in this concept because not a Cannon.
Also it can
not simply be built. It is just a rare
Special Event Unit.
(I personally have no intentions att all to change any of the Special Event Units to Professions.)
We will not touch Ships or Wagon Trains or ... either.
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@team:
Please let us clarify if it is ok or not for you or if there was a misunderstanding.

We need to do that
before Schmiddie starts putting effort into collecting / preparing his old graphics.