Shifter is an old power classification recently collapsed into  Changer before the start of Worm. The two terms are now used interchangeably,  both in speech  and in written documents. 
When Shifter was still in active use, it referred to a parahuman that could alter their appearance, while
Changer referred to a parahuman that could physically take on another form. Both  Biter and Oliver are examples of Shifters as defined under the older classification. 
↑ Other classifications have existed and been folded into the ones we have now. Shifter is a recent collapse. -
reddit comment by Wildbow. ↑
2.0 2.1 The PRT used to have far more than the 12 classifications, but reduced them down. Shifter was folded into Changer and the two are now used interchangeably. -
reddit comment by Wildbow.
↑ “Maybe later. For now…” the accented male voice spoke, “Just satisfy my curiosity. You know the passwords we memorize each week, and you know why we memorize them, right?”
“Yeah,” Shadow Stalker replied.
One of the other boys spoke, “For any interaction with any flagged shifter or,” the boy paused, “master. Oh.”
“So,” Weld said, “Keeping in mind that Regent is the highest rated Master in the city, I’d like for you to give us this week’s password.” - Excerpt from Parasite 10.2
↑ Blasto, Real Name Unknown
Classification: Tinker 6 (sub: master 5, blaster 2, shifter 2, brute 2); plants.
Disposition: Villain (B)
Last Known Location: Boston (Allston area, east). - Excerpt from Interlude 19 (Donation Bonus #1) ↑
5.0 5.1 Someone who could alter their appearance to look like another person/alter their human features vs. someone who could physically take on another form.
Biter would have likely been a shifter vs. a changer. - reddit comment by Wildbow. ↑ She’d be a Trump/Master hybrid.
Oliver would be a low-level shifter. - Cell 22.6, comment by Wildbow.