I have a pair of M-Audio BX5a speakers, one of the LF driver is fried so now I am thinking if I can upgrade to an 8 ohms driver.
The speakers is an active 2 way with 30watts for the 6 ohm tweeter and 40watts into 4 ohm mid bass driver.
Why do you think going to 8 ohms is an "upgrade"?
You will cut LF power by half
Since it's a Studio monitor and accuracy is most important, I'd first try to get an original replacement from Manufacturer, and if not possible, would search for an as-equal-as-possible replacement and replace *both* at the same time.
These active cabinets are heavily equalized internally to get as flat as possible response; even fitting a "better" speaker there will not do, because compensation will be way off.