If you're only getting a $15 discount off the regularly $40 line access fee, then YES it would be beneficial for you to switch. Only because you'd then be paying exactly the same and getting 1 extra GB.

$45 - 3GB of data

$xx - Monthly device payment amount

$20 - line access fee

The edge discount goes away and is automatically set to the new $20 and it seems right now you are paying $25 for your line access fee. Is that correct? So it would drop $5, but then the plan would increase $5.

Now if you have other lines on the account to consider this might change things.