When learning a foreign language, it is important to review. The saying goes, "If you don't use it, you lose it."
Just like when you learn to play an instrument or a sport, or many other things, you need to keep your skills polished in order to maintain your level of expertise and to go beyond.
If your student(s) use Spanish for You! you should always keep your old books and downloads so they can be reviewed. Once or twice a week, read one page from the old book with the audio just to jog the memory.
For example, let's say you have used our Fiestas package, and now you are using our Viajes package. During your weeks of Viajes, once or twice a week, open the Fiestas book and read a page with the audio. Redo a worksheet too if you have the time!
The reason I suggest this is that some material overlaps from book to book, but not all. So, you want to do some maintenance work to keep those things not overlapping from being forgotten. Also, this helps enrich your student's experience using the newer package.
I have many students who will ask me if they can add in verbs or vocabulary or grammar concepts from a past book while doing activities in the package we are currently using. They love to do this, and I see it really motivate them. They really get excited about how much they know and are able to do!
So, be sure you take a little time each week to review. Just 5 minutes a couple of times a week will do wonders.