Change bottle nipple speeds?

Mrs.AMB07

I know slow nipple is most like breast feeding, but I'm wondering if it's "okay" to change DD to medium flow. She gets one bottle a day for practice and so DH can feed her.

 She's started to take only the first 2 oz of her 4 oz bottle. Then she gets upset. She wants the bottle and then cries when she tries to drink. We thought it might have been teeth but we tried a medium flow and she did beautifully.

Since giving up the nursing shield last month nursing sessions have gone from 40 min down to 12-15 minutes total. Could she now be frustrated with the slow flow? Would it hurt her nursing to go to mediums?.