OhDammm

I am wondering if it is worth the money to upgrade my 528i or just trade it it for a 535 or a 550.

Boost7

if you want more power, then get something else. if you just want cosmetic upgrades, then keep it.

bapeforceone

If you're really looking for more power, NA upgrades won't do a thing. Just trade in for a 535i if you really need that power. Otherwise I find my daily commute only needs the 528xi. My 335i is fun on the summer days where you're just cruising and tunes are extremely cheap and cost-effective compared to any NA upgrade you do.

If anything you may be able to swap the intake manifold + tune from a 530i I think? I believe it was swappable for 530i > 525i, not sure about 528 though. I'll let the others chime in on this if they don't flame you for searching. To be fair, this topic has been covered many times.