Firmware V1.19

[ehr]

The main reason is that this is the only way to stay in sync with everything else. So you can switch while playing to other (even) scales and it would stay synced with the possibility that you as well can switch back to normal clock. If you could do this on the fly, you would directly loose the synchronisation.
The same for the groove indeed, if you edited the groove and the sequencer is passing half waym then your chance it big that it would run out of sync (because thats what is programmed in then).

That makes sense. 

The way I've been using groove column is to double the ticks per step of all the steps in the patterns that I want to play at a /2 speed then press play.   I don't make changes while the pattern plays.   I just thought that putting one clock command on the first step of the pattern would be easier than changing all the groove steps of the pattern.   No worries using groove works good.

But when that track is not playing, the scale changing will take effect directly, pressing start then will start the pattern directly in the chosen scale.

Goove works this way, but SCLK in the effects column doesn't.  It takes effect after the pattern loops around once or on the start of the next pattern if there is a pattern chained after it.

Just to be sure: You can change a non playing track scale which will take effect immediately (using the Nerd button)

Yes, but I like to use the groove column because I have multiple songs in each project and prefer to have the setting per song, with groove, and not have to change the track scale between songs.   Would you consider having SCLK command scale immediately like the Nerd button track setting if pressing Play from a stopped sequencer, and if the sequencer is running have SCLK function the way it does now?  Just a thought, not important...we have groove