I'm wanting to get a new fork for my bike but I have a sealed bearing hub (not sure what axle size, 5mm or 9mm) I was just wondering can I convert a sealed bearing hub to a different axle size? (15mm or 20mm thru axle)
Depends totally on your hub.
If its currently a through-axle, then the fork has to support the same diameter.
If its currently a QR, take the wheel off, remove QR and see if there are inserts that can be popped off to show a larger hole. If no, you'll need a new hub. If yes, you need the two adapters, which might be hard to find.
A new wheel might be more cost efficient than a new hub+wheel rebuild.
Also, the warranty conditions on your fork may preclude the use of any adapters - best check that too.