One of the professors that I was planning to ask for a letter of recommendation told me that she was moving to the other side of the country at the end of this school year. Could I ask her for a letter of recommendation now and just let the letter sit in my CAS until I need it when I apply next cycle?

I am a current junior, planning to apply during next year's cycle (October, 2018). I think I saw online that letters of recommendation are valid for up to five years, but I was just worried that the date difference between when I receive the letter and when I use it may affect my applications somehow.

Does anyone have any advice or insight on my situation? Thank you in advance!