Upgrading SSD on 2015 MacBook Pro
Is it possible?  

Yes.

The question is can you get a drive to upgrade to?

Apple uses a "proprietary PCIe 2.0 x4" interface for their storage.  There are SSDs out there that match the specs (like this Samsung on Amazon), but for some reason they are not compatible

So, bottom line....

Yes, you can upgrade it, but only with a genuine Apple SSD (at this point).  Your best bet is to get the most you can afford now because there is zero cost savings plus the added headache of disassembly/reassembly.  IMO, it just isn't worth it.