It seems that undo is not currently implemented when changing parameters from VST(I) GUI. I know that using renoise’s own parameter tweaking enables undo but I use GUIs much more often. How hard would this be to implement?
On the gui itself impossible because the VST plugins are applications of their own.
But maybe parameter changes can be polled by Renoise and stored in memory and some sort of A/B editing would probably be possible as well.
In this case the routine in the plugin should be called as if the song would be saved.
I don’t know what Taktik finds of this idea… we have A/B editing in the FX chain, why not on VSTI level either (two extra buttons beside the “Enable Keyboard” checkbox)
Saving presets or banks is already possible.
Sounds great and useful indeed.
A/B editing for VSTi - kinda nice idea.
Currently i’m copying the virtual device (with copy command from rightmouse dropdown menu), before i changing some parameters in VST(i)s… And if things go wrong and i need to restore previous settings - just pasting it back 
I agree with this suggestion too, it’s incredibly useful, but then again this was posted back in 2007 so maybe it is impossible after all?
If it can copy settings from it, then Renoise ‘knows’ what parameters is within, so why not undo?
Renoise remembering the tweaked parameters has so many benefits. Not only would we be able to do A/B testing and undos, but we’d also be able to figure out which parameters are which when trying to automate VSTs.
VV: I really appropriate that you want to help, but this is not true.
This will already work if the plugin explicitly supports this - implements this. Supporting undo/redo in the host is part of the VST standard.
So if you favorite VST does not support this, write the developers a friendly mail and request it.