Yesterday I opened an older project, where I had recorded some simple chords in a midi synth.
I had changed original synth to an organ. This new synth just does not react to my recorded chords.
It plays sound from its virtual keyboard.
I tried to "Freeze synth" which just seemed to delete the short chord sequence.
Unfreeze/Ctrl-z put it back. But it does not seem to send anything to the synth.
What weird protocols are implemented in MIDI?
Do I need to learn magic or something?

Have you checked the channel data in the copied MIDI data.  What MIDI channel is the new Synth set to?   

I did not copy any MIDI data. I just changed synth.
I'm at work, but my memory tells me "Event viewer" said Channel 10
I've tried the input-> Midi omni-dance at the strip.

What are your MIDI echo Status indicators showing?

I would have to check that as I get home. Thx, by the way. :)
I just find i weird, that I insert a synth that work. Change synth, and the sound dissappear.
No other changes. The new synth does not receive the chords I've recorded.
And sometimes it works fine. Other times I have to change some obscure parameter.

Input echo=auto thru
Button is blue.

PS! My midi keyboard is disconnected, as the chords are allready recorded..
(Allso confirming Channel 10)

I tried to insert an instance of EZ keyes, copied the track, and behold. There was sound. 
Just not in the organ.
Midi is cursed, I swear!

What happens if you transpose it up or down by an octave or two?

What is the VST that is not working? Someone else could test it. 
Normally if you use anything BUT Ch 10 for synths. Only drums should use CH 10 
If you use "Replace synth"  there should be no problem. 

Tried transposing: No change.
The "new synth" is a hammond organ: "Blue 3"
I used "Replace synth"
I did not chose channel 10. I just inserted a synth, and replaced it.