I know this a related question but we can't afford to swapping and changing in Japan- they are expensive here. Our 5 month takes one mouthful of the bottle milk and won't drink any more whether its expressed or formula- he is all breast feed but it would nice to have the option when my wife is sore from the posture she uses when normally feeding.
There are actually a few other options than just switching brands - you can use a spoon, a beaker or a sippy cup to feed your baby. At five months you either have already done or probably will soon start with solid foods and these tools are going to be part of your equipment anyway. Some exclusively breastfed babies simoly dislike baby bottles, but readily accept the alternatives above.
And even though we don't question the premise of a question here, in this case I suggest that your wife additionally re-evaluates her nursing technique. Soreness happens, yes, but some negative side effects of nursing can and should be avoided. A lactation consultant, midwife or similar might be a good first source of information.