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I was thinking of using a line like "I used to have a crush on you years ago"
I wouldn't advice you to. This telegraphes interest, and could be a DLV if you don't do it properly. Yeah, sure, if you do it C&F you can pull it off. However, if you say it in an AFC-way it will blow you out (as it will tell her that you still have, even if you really don't).
If you want to do this, I recommend the following:
1. Go into topic of, if you had any GF/BFs at kindergarten, go further in time.
2. Say, "I used to have a crush on YOU, haaa", as it was as stupid as trying to have a serious relationship at the age of 5.
3. She might tell you how she FELT, or you can ask her. Though this info is kind of old, and might not be usefull. If she liked you, you can start busting her balls about it. "Hey, when am I going to get those loveletters?".
Just my view of it.