I was searching for some New Flyer busses on the internet, and I found this:

busexplorer.com wrote
New Flyer, since 1987

On July 15, 1986, Den Oudsten of the Netherlands bought Flyer Industries from the Manitoba Development Corporation. The company was renamed New Flyer Industries. The Den Oudsten experience in bus building goes back to 1926 and the company has become the largest builder of intercity buses in the Netherlands.

If you click on the link on the comments of this bus:
http://www.imcdb.org/vehicle_48197-New-Flyer-D-800.html

It is called Flyer, since it's a 1974 bus.

Am I right when I say busses before 1986 are 'Flyer' and after 1986 'New Flyer' or do we want to make it not too complicated?

Yes

Thanks, I have changed them, there was one New Flyer without a year, but I think it's an old one.