I'm not quite comfortable with that idea, myself.
With the total uncertainty of CM training, and the uncertainty of upgrading a NE ticket...
It could be good to stop by a Guest Relations outside a park and ASK before going into the park.
If the CM indicates the (s)he would be able to price bridge if the ticket was used, use the ticket at the gate,
then come right back out to that same CM and do the upgrade.
OTOH, I'm also not so sure if upgrading is the best use of your current ticket, anyway.
I don't have all of the particulars to get a better handle on things.
You've not said how many days are on the NE ticket.
If you will make several trips to WDW over the years,
it might be wise to just keep the benefits of having a non-expiring ticket
and straight-purchase an AP for use this year.