Buy some filters and filter the gear. If it's a UG lab, then it would be typical if you got an infection from one vial to get an infection from a different one. Most UG's in the states are made in some guys basement or garage...not exactly lab conditions. Filter it, add BA, heat it up in the oven are all ways to help sterilize it.

Changing needles after you draw to inject is only a personal preference. The needle doesn't dull to any significant amount to warrant it and also, if you are wiping the top of the vial with alcohol it won't get contaminated to warrant it either. Ask any doctor or nurse if they change needles before injecting... "No"

Only reason I can see for this practice is if you have thick gear and want to draw it up fast to inject with a much smaller gauge needle.