Of course, e.g. PG. *ē becomes ON. á: *knēaną → kná.
There is no inherent restriction on which path a sound shifts, towards the front, towards the back, higher, lower, anything is possible. It all depends on the underlying phonological structures and processes.
As I mentioned earlier, it's not so much the sound change as the underlying process that is important. Can /e/ shift to /a/? Yes, but that's unimportant. WHY (if so) does /e/ shift to /a/, is what is important.
Then again, I am kind of generalising (a lot), but the underlying processes are important as sounds can change for many reasons, and it's important to distinguish between the underlying reasons.
To put it in a medical proverb: study the disease, not just the symptoms.