It's doable, but it's definitely an involved job and not something you would do without considerable soldering experience. I will never gatekeep working on pedals and will always be around to provide guidance if someone wants to take a swing at it, but I'm also always happy to do the work for you if you're not sure you can handle it.The 6N21B and 6N17B tubes are still available and they are cheap, there is absolutely no reason why the tubes could not be changed in any early Sushi Box pedal but it is a solder job and not just plug into a socket job. Any pedal with those tiny tubes can be worked on as long as you can solder and know what you are doing, or take it to any good amp tech and he should be able to do it.
At least that is my opinion but I have never worked on any early Sushi Box pedals.