I have an old front hub with what I believe is a 5/16" axle in it and I want to use it on a build but the axle is too small for the fork ends. I need an axle sized for a track hub. Is it possible to change the axle out for a larger one?

Need to know what kind of hub you have now, but in general, NO. Hubs and axle sets are generally made as sets.

The smaller axle hubs (Wald and similar) usually use a specific washer to center the axle in an oversize hole. At least that's the way it was in my Schwinn days.

Sometimes, but usually not. The limitations include the availability of cones that fit, and in many cases the clearance of the center of the hub. Start by removing the 5/16" axle and seeing if a 3/8" rod will slip through the hub, if yes, then you can start hunting for cones.

OTOH, there's usually no problem using a 5/16" axle woth a fork whose dropouts are sized for a QR axle. To ensure that the wheel always goes back on the same way, get in the habit of tightening the nuts with the bike vertical on the ground. That will let gravity position the wheel consistently.

If you still want a tighter fit, the easiest way is to make a pair of rings to slip over the axle inside the dropouts. Years ago I made a pair by cutting them off a piece of 3/8" OD copper curtain rods. They took about 10 minutes with a saw and file to make. Just be sure that they're narrower than the dropouts so they don't keep the nuts from tightening enough.

It's a NOS loose ball high flange Sunshine BMX front hub. The same thing as the one in the pic

I wanted to use it in a fixed gear freestyle build. I remember back in my BMX days i could change out 3/8 axles for a flat 14mm axle on BMX hubs that's why I was wondering if I could do it from 5/16 to 3/8.

Oohhh... maybe. I remember those hubs came in both 5/16 and 3/8 axles. If you can find a 3/8 axle set (axle, cones, locknuts, etc.) you're home free. I like FB's solution, though- much simpler.

Thanks for all the help.