Upgrading to an MK3.9S only really makes sense if the existing MK3S/+ still works, but the extruder/hotend is broken. I have a few cases like this where it would be necessary to completely overhaul the entire hotend/extruder component. They're simply worn out.
It's probably too late now, but what I would have liked would be a pure extruder upgrade. That is, just replacing the existing extruder with the Nextruder. Of course, I can't say whether that's possible with all the other changes.
I only upgraded to the MK3.9S because it was the only upgrade package that also included a replacement extruder. I don't really need the improved print quality, nor the increased speed. But I do need the significantly improved quality of the Nextruder.
It's probably too late now, but what I would have liked would be a pure extruder upgrade. That is, just replacing the existing extruder with the Nextruder. Of course, I can't say whether that's possible with all the other changes.
I only upgraded to the MK3.9S because it was the only upgrade package that also included a replacement extruder. I don't really need the improved print quality, nor the increased speed. But I do need the significantly improved quality of the Nextruder.