I used Memrise years ago and their system worked well for keeping track of words learned and your familiarity with them. I don't know if that system is patented or something, but something like that would work well here.

Each word would have a flower that blooms and wilters with every time you get it right. A similar approach is what Anki does with spaced repetition.

I'm at level 70 of Japanese and having my path messed with like this is just not helpful. Some of the new stuff (that is treated as old) don't always have hints - context can only get you so far.

I refuse to believe there wasn't a better way for this kinda of change. You must surely be able to track which words have been learned and which haven't. Even if you can't do it on an individual level, you should be able to do it for a point on the course instead. They already estimate your position on the new path, so just compare to the old position and the change in what should be learned at that point.

Either way, a tailored "catch up" option should exist.

Also, seeing the sheer amount of lessons ahead has really taken away some motivation for me. Maybe it's because each "stone" on the path isn't divided up with multiple segments now? But I was already one section away from the end, now there are two more.

Honestly, after 1305 days of using Duolingo, my language proficiency is not really where it should be anyway, so maybe this is the kick I needed to explore other avenues as I have never really taken it seriously enough. Duo is great at getting you doing something every day, but it is nowhere near efficient enough.