Using miles to upgrade constitute using a confirmed instrument (if NC or NF > 0, you will get upgraded immediately regardless of time before the flight). e-500s are only confirmable at the earliest 100hrs assuming you are GS. GA or CSR should never switched a confirmed upgrade with a confirmable instrument with e-500s, it is extremely unfair.
A GS or 1K will use his/hers confirmable upgrades early on and switch to e500 close the flight time and no one else will be able to get any upgrades.