I'm pretty sure that you can't port a number from 48 to 48 - or from any carrier to the same carrier.
But I've never tried it.
I presume that the 48 porting form won't list 48 as an option for the "from" carrier.
You might want to ask 48 support.
See the link in my signature below or email them at firstname.lastname@example.org
I wrote to the support through their email and got this reply: "I'll surely check your account and help you with your query." and asking for account details.
Not sure if that is a yes, and after I provide my details they will do it or just standard procedure to ask account details and then say no.