You can merge the two line under one account, so your girlfriend can do a transfer or responsibility of her line to you so both lines will be under one account.
In regards to the plan, if the plan is still available then it is possible you can get the same plan but if the plan is a grandfathered plan or a loyalty plan then you will not be able to get the same plan.
If you do not like the plan you currently have to can always call Fido and see what other options you have, sometimes they have targeted/loyalty plans just for you.
KAPABLE-K is correct on all point here!
I just wanted to mention that if her plan is still offered, you can actually change your plan yourself through My Account on Fido.ca. Check out how to do so here.
If her plan is a loyalty plan, and your line is eligible for the same offer, you will see it there.
Otherwise, sadly, you would need to select from one of the other plans we offer.