I took the freedom to update your post @Marco_Weiss, instead of
plan_upgrade it's now simply
resize both for listing available upgrades and to perform the upgrade.
The exact same syntax can now also be used to change the image by using
rebuild, keep in mind that this will overwrite anything that is currently on your server. If you instead wanted to repair a broken server instance you'll have to use
rescue. Examples below:
Resize: Works on all generations now, G2 may also get support for down scaling by giving the option to not resize the disk during upscale. This would be represented as an additional boolean value and will be added to documentation once included and working.
Rebuild: Works on all generations,
image_id can also be uuid strings.
Rescue: Works on G3 only. Work is in progress for G1 and thereafter G2 by using custom built rescue images.