I looked at doing a 'backwards' differences course from Global Vision to Global Classic (Honeywell) - In EASA land this is absolutely not possible (Verboten) and I would have to have done a full Global Classic Initial.
In FAA Land, you can do a 'backwards' differences course between Global Vision and Global Classic, but you can then only fly an FAA registered aircraft and cannot then subsequently transfer it across to an EASA Land aircraft.
In terms of holding both Type Ratings i.e.; Classic and Initial - I have never seen anything written down but I recently did some work business with the Bermudan Authorities for a Challenger pilot who held both 604 and 605 Type Ratings, and they would not validate him on the 604 if he had not done a full 604 Recurrent in the previous 2 years - they also pointed out to me that this would be the same for the 605 - ergo, if you want to satisfy the Bermudan authorities and probably some of the others - it seems sensible (and dare I say it at the risk of being verbally abused and mangled) and professional to do one every other year
However, I suspect some people's Ops Manuals will say you have to do one each year and therefore you will have to do 2 Recurrent's each year for both Classic and Vision - and if you move onto the 7000/8000 then you could be onto 3 a year!
At that stage you've probably been flying far too long and its time to give up!