Hi there and
to the forums!
Generally, when introducing new foods, it's best to do it gradually so as to not stress them. You can purchase a bag of higher quality seed and ask for a small bag of the food they feed them to transition, or if they don't do that, buy a small bag and then gradually mix it, increasing the new seed an decreasing the old seed so he is used to it.
If it's just a basic seed mix, however, (nothing specialised, etc.) then I'd reckon you could just buy that and as he settles into his new home he will also adjust to the new seed. This is what I did when I brought my little girl home since they were feeding her just a basic seed blend and I got a better quality one with the same basic seeds and she was just fine
Do you know which brand they are feeding him at the pet store?
Meanwhile, I'm glad you've joined the forums--how exciting you'll be getting your budgie soon! It's great you're doing your research first, that is wonderful. There's lots to read around here, so be sure to look through the Budgie Articles as well as the "stickies" (important threads "stuck" to the top of each subforum for easy reference) to ensure you're up to date on all the best in budgie care and practices!
Feel free to ask any other questions afterwards, if you have them!
Hope to see you around!