Basically as above, do you lose the old membership rate if you have an older number that you did not switch prior to a rate change and want to do so now.
I don't think so.
But maybe check with 48 support to be sure.
See the link in my signature below or email them at firstname.lastname@example.org
I'm pretty sure that just porting a number in will not bump you off the old €7.99 membership, but you might want to check just to be 100% sure... :-)
Judging by this, the only thing that would bump you off being able to buy the €7.99 membership would seem to be if you bought another membership at some point.