Sokka said:
Nappa can increase his Battle Level, isn't that pretty obvious at this point?
Well I'd say people has good reasons to believe he couldn't increase his battle power, due to his shock due to Gohan changing his battle power at will (as if it was something new to him) or Vegeta saying he learned the control over his power level on Earth. But I feel that's heavily implied both of them actually did it on Earth. It's just consequence of AT's poor writing again, I guess.
Like with Freeza, it was apparently shown he doing a power up in his 1st form, with Kuririn even saying he didn't believe his power was so much:
But later Vegeta showed surprise towards 2nd form Freeza changing his battle powers, implying 1st form Freeza didn't do it.