style question

I'm working on this story I started last week for a stargateland challenge (didn't finish it in time) but I've run into a problem and not sure how to work this.

Can you start a story with "once upon a time" and then write it in present tense?

Because I started it in past with the whole "once upon a time" (so right there it's taking place a long time ago) but after the first paragraph I realized it went to present tense.  I started typing up what I had written and since there was this disagreement with the tenses I tried to change the entire story to past tense but... it's not working.  It sounds so much better in present tense.

When I'm reading it to myself it makes perfect sense for the first paragraph to be past tense, setting up the story but the rest told as if you are right there in present tense (there's also an element of 2nd person with the narrator speaking directly to the reader).  But I'm not sure if other people would appreciate that or think I was just dumb and can't get my tenses straight.

My brain is just fried, apparently, because I just can't figure this out.