Edit - based on this additional information, via email:

Basically it was fine and one day it won't shift. Only first and reverse. Stays in first as far as 4k RPM and will slip into neutral when you let off and coast. Step on accelerator and will kick into first when the revs get to around 3K. Adjusted Bowden per forum, fluid not burned and at proper level, 12mm below low level, for a cold tranny. Disconnected vacuum line to tranny, no difference.

What's described above is more than a stuck kickdown valve. It shouldn't slip into neutral, it shouldn't need to rev to 3k before kicking into first. Bowden is absolutely irrelevant, you could disconnect it if you want. A total shot in the dark is a failed front pump, but if that's not it, you're looking at a tranny rebuild. This didn't happen shortly after a fluid & filter change, did it?