Definitely start the challenge.

You will practice thousands of ghosted lines, super imposed lines and of course boxes. Add the warm up to it, and you see that there's no reason to go back.

Maybe read the material again to refresh your memory. This course is dense with information.

I was in a similar situation not too long before, with the exception that I stopped at the last steps of lesson 1 and not before the challenge. And because it wasn't too much, as well as there was new videos added when I came back (some new lesson 0 vids), I was able to review everything and start anew, so that it only took me a few days until I was the point where I stopped.

In your case, however, since the lesson 1 exercises are mainly there for warmup and "getting started" reasons, I'd say the same like Liska and recommend you to continue where you left off. Muscle memory stays for way, way longer than just a few months. You'd need to get used to it again, but it for sure won't take long; that's what I know for sure ever since I quit Badminton for a year and still got all the moves and techniques I learned even just closely beforehand.