We had to do this at 11 weeks with our daughter so I could start taking chemo.
She had no problems taking to it honestly. I think if they're not given the option, they'll quickly adapt. Formula and breastmilk apparently do taste much different (never brought myself to taste), but some of the recommendations I've heard are to mix in quantities of varying ratios heavier on the breastmilk:formula initially then heavier on formula:breastmilk.
Different formulas have different tastes, so you may have to try a few before you find the one that's best.
Call your pediatrician - they may have samples/coupons and may also have suggestions on how to make it easier. Also, I'd get their advice on which brand they prefer. Our pediatrician's only comment was that he could guarantee the claims of the name brands because they've invested in the clinical research to back-up their claims and have the studies to support them. Generic products do not, and though the ingredients may be in the same order, they may not be in the same quantities making each slightly different.
In the end, we opted for Kirkland's formula because of price and our daughter consistently being in the 90%+.
Good luck.